Firstly: 

This incident was narrated from the Messenger of Allaah (peace and blessings of Allaah be upon him) in the books of Sunnah. To sum up, the people had formed their rows to pray behind the Prophet (peace and blessings of Allaah be upon him), and he stood before them as if he was going to lead them in prayer, then he remembered that he was junub, so he gestured to them to stay where they were, and he went and did ghusl, then he came back to them with his head dripping with water. 

But there are some differences in the reports: 

Did the Prophet (peace and blessings of Allaah be upon him) say takbeer and then remember that he was junub? Or did he remember before the opening takbeer? 

The reports may be summed up as follows: 

1 – A version which clearly states that he remembered that he was junub before he started to pray: 

This version was narrated by ‘Abd-Allaah ibn Wahb from Yoonus from al-Zuhri from Abu Salamah ibn ‘Abd al-Rahmaan ibn ‘Awf from Abu Hurayrah (may Allaah be pleased with him), as was narrated in Saheeh Muslim (605), where it says: The iqaamah for prayer was given, and we stood up and made the rows straight before the Messenger of Allaah (peace and blessings of Allaah be upon him) came out to us. Then the Messenger of Allaah (peace and blessings of Allaah be upon him) came and stood in his prayer place before saying the takbeer, then he went away and said to us: “Stay where you are.” We remained standing, waiting for him, until he came out to us. He had done ghusl, and his head was dripping water. Then he said takbeer and led us in prayer. 

Saalih ibn Kaysaan narrated from al-Zuhri – as was recorded by al-Bukhaari (639) as follows: 

… until, when he was standing in his prayer place and we were waiting for him to say takbeer, he turned and said, ‘Stay where you are.’ We stayed where we were until he came out to us with his head dripping with water, as he had done ghusl. 

2 – A version which clearly states that he said takbeer and started to pray, then remembered that he was junub. 

This was narrated by Usaamah ibn Zayd from ‘Abd-Allaah ibn Yazeed, the freed slave of al-Aswad ibn Sufyaan, from Muhammad ibn ‘Abd al-Rahmaan ibn Thawbaan, from Abu Hurayrah (may Allaah be pleased with him), as is recorded by Ibn Majaah in al-Sunan (1220), al-Daaraqutni (1/361) and al-Bayhaqi in al-Sunan al-Kubra (2/397). This version says:

The Prophet (peace and blessings of Allaah be upon him) came out to pray and said the Takbeer, then he gestured to them to wait. He went and did ghusl, and his head was dripping with water whilst he led them in prayer. When he finished he said, “I came out to you in a state of janaabah, and I forgot until I had started to pray.” 

But there is some weakness in this isnaad with regard to Usaamah ibn Zayd al-Laythi, Abu Zayd al-Madani. In his biography in Tahdheeb al-Tahdheeb (1/209) it says: He was regarded as matrook by Yahya ibn Sa’eed. Ahmad said: he is nothing. Abu Haatim said: His hadeeth may be written down but should not be used as evidence. Al-Nasaa’i said: He is not strong. Ibn Hibbaan said: He makes mistakes although he is of sound character. End quote. No one narrated that he was trustworthy except Yahya ibn Ma’een, but in another report from him he said that his ahaadeeth were regarded as munkar. 

This version was also narrated via Muhammad ibn Sireen from Abu Hurayrah (may Allaah be pleased with him), that the Prophet (peace and blessings of Allaah be upon him) said takbeer, leading them in Fajr prayer, then he gestured to them and went away, then he came back with his head dripping, and led them in prayer. He said: “I am only human; I was junub and I forgot.” 

Narrated by al-Tabaraani in al-Awsat (5/317) and al-Sagheer (2/74). He said: No one narrated it from Ibn ‘Awn except al-Hasan ibn ‘Abd al-Rahmaan, and it was narrated only by Abu’l-Rabee’ al-Haarithi. It was also narrated by al-Bayhaqi (2/398). But this report contains an error. The correct version was narrated from Muhammad ibn Sireen from the Prophet (peace and blessings of Allaah be upon him). That is based on two points: 

1 – The trustworthy narrators among the companions of Ibn ‘Awn differed with the report of al-Hasan ibn ‘Abd al-Rahmaan. Al-Bayhaqi said: It was narrated by Ismaa’eel ibn ‘Aliyah and others from Ibn ‘Awn from Muhammad from the Prophet (peace and blessings of Allaah be upon him) in a mursal report.  End quote.  

2 – Others followed Ibn ‘Awn in his mursal narrations. Abu Dawood (233) said: It was narrated by Ayyoob, Ibn ‘Awn and Hishaam from Muhammad in a mursal report from the Prophet (peace and blessings of Allaah be upon him). End quote. 

Al-Bayhaqi said in Ma’rifat al-Sunan wa’l-Athaar (1306): This is how it was narrated by Ayyoob and Hishaam from Muhammad via a mursal isnaad, and it was narrated by al-Hasan ibn ‘Abd al-Rahmaan al-Haarithi from Ibn ‘Awn, from Muhammad, from Abu Hurayrah, but the former is more sound. End quote. 

There is some corroborating evidence in other reports which say that the Prophet (peace and blessings of Allaah be upon him) remembered he was junub after he had said the takbeer and started the prayer: 

1-

It was narrated from al-Hasan from Abu Bakrah (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) started to pray Fajr, then he gestured with his hand, saying stay where you are, then he came with his head dripping and prayed. 

Narrated by Abu Dawood (233), Ahmad (5/41), Ibn Khuzaymah (3/62), Ibn Hibbaan (6/5), al-Bayhaqi in al-Sunan al-Kubra (3/94) and al-Tahhaawi in Bayaan Mushkil al-Athaar (2/86). According to some versions it says “and he said takbeer”. 

Ibn Rajab said in Fath al-Baari (3/599): The hadeeth of al-Hasan from Abu Bakrah is like a mursal report, because al-Hasan did not hear from Abu Bakrah according to Imam Ahmad and the majority of earlier scholars. End quote. 

2-

It was narrated that Anas ibn Maalik said: The Messenger of Allaah (peace and blessings of Allaah be upon him) began to pray, and he said takbeer and we said takbeer with him, then he gestured to the people (as if saying) stay where you are. Then we remained standing until the Messenger of Allaah (peace and blessings of Allaah be upon him) came to us having done ghusl, and his head was dripping with water. 

Narrated by al-Daaraqutni (1/362), al-Tabaraani in al-Awsat (4/92), al-Bayhaqi in al-Sunan al-Kubra (2/399) via Mu’aadh al-‘Anbari from Sa’eed ibn Abi ‘Aroobah from Qataadah from Anas. 

This hadeeth was also narrated in a mursal report from Qataadah from Bakr ibn ‘Abd-Allaah al-Muzani. 

3-

It was narrated that ‘Ali ibn Abi Taalib (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) led us in prayer one day, and he left, then he came with his head dripping water and led us in prayer. Then he said: I led you in prayer just now when I was junub. Whoever experiences something like I experienced, or feels cramps in his belly (as if needing to defecate), let him do what I did.” 

Narrated by Ahmad (1/99), al-Tabaraani (6/272) and al-Bazzaar (890). Its isnaad includes Ibn Luhay’ah, who is somewhat weak. Al-Tabaraani said in al-Awsat (6/272). This hadeeth was only narrated from ‘Ali with this isnaad, and it was narrated only by Ibn Luhay’ah. 

4 – It was narrated from ‘Ata’ ibn Yasaar that the Messenger of Allaah (peace and blessings of Allaah be upon him) said takbeer in one of the prayers, then he gestured to them with his hand to stay put, and he went away, then he came back with traces of water on his skin. 

Narrated by Maalik in al-Muwatta’ (1/48/no. 110) with a saheeh isnaad to ‘Ata’ who was one of the Taabi’een. 

Al-Bayhaqi said in al-Sunan al-Kubra (2/398): 

The report of Abu Salamah from Abu Hurayrah (may Allaah be pleased with him) is more saheeh than the report of Ibn Thawbaan from him, but with  the report of Ibn Thawbaan from him there is the report of Abu Bakrah, and the report of ‘Ata’ ibn Yasaar and Ibn Sireen is mursal. It was also narrated from Anas ibn Maalik. End quote. 

To sum up: When all these many isnaads are taken together, which report that the Prophet (peace and blessings of Allaah be upon him) said takbeer and started to pray before he remembered that he was junub, especially the saheeh reports from Ibn Sireen, Bakr al-Muzani and ‘Ata’ ibn Yasaar, all of that indicates that the incident did indeed take place, and there is no contradiction between these reports and the report of Abu Hurayrah which says that it happened before he said the takbeer, because the incident may have been repeated, so it happened in one way on one occasion and in another way on another occasion. Thus it is possible to reconcile the reports.  

Abu Haatim Ibn Hibbaan said in his Saheeh (6/7): 

This is something that happened twice, on separate occasions. The Prophet (peace and blessings of Allaah be upon him) came out on one occasion and said takbeer, then he remembered that he was junub, so he left and did ghusl, then he came back and resumed the prayer; and on another occasion he came out and he stood to say takbeer, then he remembered that he was junub before he said takbeer, so he went and did ghusl, then he came back and led them in prayer, and there is no contradiction between the two reports. End quote. 

Al-Nawawi said in Sharh Muslim (5/103): 

It may be that they were two separate incidents, which is more likely. End quote. 

There is no problem, praise be to Allaah. The Prophet (peace and blessings of Allaah be upon him) was a human being and he forgot as all people forget. He made mistakes in his prayer on many occasions, and none of the scholars denies that, rather they said that he was infallible and protected from forgetting anything of the wahy (revelation) completely in such a way that would cause it to be lost. 

Secondly: 

With regard to the fiqh of the hadeeth and deriving rulings from it: 

It indicates that if the imam leads the people in prayer when he is not in a state of purity (tahaarah) by mistake, then the prayer of the people praying behind him is not affected, and it remains valid; and the imam is the only one who is enjoined to repeat it, not those who prayed behind him. 

The reason for that is that the Prophet (peace and blessings of Allaah be upon him) went back and did ghusl, then he came and said takbeer for his prayer, and the people behind him were in their rows and were still in a state of prayer, and he did not tell them to repeat the takbeer with him. 

Ibn Rajab said in Fath al-Baari (3/600-602): 

There is nothing in the hadeeth to indicate that the Prophet (peace and blessings of Allaah be upon him) continued from where he had left off from the opening takbeer when he forgot that he was junub, so it could only have been one of two scenarios: 

1 – When he (peace and blessings of Allaah be upon him) came back he said the opening takbeer and the people said takbeer with him 

2 – or the Prophet (peace and blessings of Allaah be upon him) repeated the opening takbeer, and the people behind him continued from their previous takbeer. 

This is what was suggested by al-Shaafa’i, who regarded it as the basis for the validity of the prayer of one who is in a state of purity behind an imam who prays in a state of impurity because he  forgot. 

Ibn ‘Abd al-Barr said: some of the companions of Maalik agreed with al-Shaafa’i concerning that. 

Two reports were narrated from Imam Ahmad concerning members of the congregation starting and completing the prayer if they follow one who forgot that he was impure, then he remembers it during his prayer. 

It was narrated from al-Hasan that they should complete their prayers. 

The view of al-Shaafa’i is that it makes no difference whether the imam forgot that he was impure or he remembered it, if the person who prayed behind him did not know, and the one who prayed behind him does not have to repeat his prayer. 

This is the view of Ibn Naafi’ among the Maalikis, and Ibn ‘Abd al-Barr narrated it from a number of fuqaha’ in the regions and scholars of hadeeth. 

And it was narrated from Maalik and Ahmad that the one who prayed behind the imam has to repeat the prayer. 

Hammaad, Abu Haneefah and his companions, and Sufyaan al-Thawri –  according to the better known report from him –  said that the one who prayed behind the imam should repeat the prayer, even if the imam forgot and did not remember until he completed his prayer. End quote. 

On our site you will find a number of answers which state that the prayer of the one who prayed behind an imam is valid if it becomes apparent that the imam is without wudoo’, and that it is only the imam who has to repeat the prayer, not those who prayed behind him. See 27091 and 85011. 

And Allaah knows best.