Firstly: 

The one who loses his wudoo’ during the prayer, by passing wind or otherwise, should go and do wudoo’, then come back and start the prayer from the beginning all over again, according to the more correct of the two scholarly opinions. This is the view of the Maalikis, Shaafa‘is and Hanbalis, unlike the Hanafis and the older view of the Shaafa‘is. 

This applies to the one who cannot help breaking his wudoo’. As for the one who breaks his wudoo’ deliberately, his prayer becomes invalid according to scholarly consensus. 

The evidence quoted by the majority concerning this issue is analogy (qiyaas). They say, Because breaking wind and the like makes wudoo’ invalid, so it also makes the prayer invalid, as is also the case when one breaks wudoo’ deliberately. 

The Hanafis quoted as evidence the hadeeth of ‘Aa’ishah (may Allah be pleased with her) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever regurgitates, has a nosebleed, vomits, or emits madhiy (prostatic fluid) should stop praying, do wudoo’, then resume his prayer, and whilst he is in that situation he should not speak”, but this is a da ‘eef hadeeth. It was narrated by Ibn Maajah, 1221. al-Busayri said in az-Zawaa’id: Its isnaad includes Ismaa‘eel ibn ‘Ayyaash, who narrated from the Hijazis, but his narration from them is da‘eef (weak). 

Al-Haafiz ibn Hajar said in at-Talkhees al-Habeer, 1/495: More than one of the scholars criticized the report of Ismaa‘eel ibn ‘Ayyaash from Ibn Jurayj and the report of Ismaa‘eel from the Hijazis as da‘eef. The scholars of hadeeth among the companions of Ibn Jurayj differed with Ibn Hajar and they narrated it from Ibn Jurayj, from his father, from the Prophet (blessings and peace of Allah be upon him) in a mursal report.

End quote. See: at-Tahqeeq fi Ahaadeeth al-Khilaaf by Ibn al-Jawzi, 1/83; Tanqeeh at-Tahqeeq by Ibn ‘Abd al-Haadi, 1/284 

An-Nawawi (may Allah have mercy on him) said: We have stated that our view is the correct, new one, that it is not permissible to resume (from where one left off); rather one has to start all over again. This is the view of the Sahaabi al-Miswar ibn Makhramah (may Allah be pleased with him). It was also the view of Maalik and others. This is the correct view from the madhhab of Ahmad. 

Abu Haneefah, Ibn Abi Layla and al-Awzaa‘i said: He may continue his prayer from where he left off. This was narrated by Ibn az-Zabbaagh and others from ‘Umar ibn al-Khattaab, ‘Ali and Ibn ‘Umar (may Allah be pleased with them). It was also narrated by al-Bayhaqi from ‘Ali, Salmaan al-Faarisi, Ibn ‘Abbaas, Ibn ‘Umar, Ibn al-Musayyab, Abu Salamah ibn ‘Abd ar-Rahmaan, ‘Ata’, Tawoos, Abu Idrees al-Khawlaani, Sulaymaan ibn Yazaar and others (may Allah be pleased with them). The author mentioned in brief the evidence for both views, but the hadeeth is da‘eef. The Sahaabah (may Allah be pleased with them) differed concerning this issue, so the matter may be resolved on the basis of analogy. And Allah knows best.

End quote from al-Majmoo‘, 4/6 

See also al-Mughni, 1/421 

Ash-Shaafa‘i (may Allah have mercy on him) interpreted the reports about some of the Sahaabah exiting the prayer because of nosebleed, then going to do wudoo’, then resuming the prayer from where they left off as referring to washing away the blood, not wudoo’ in the sense of ablution for prayer. 

And he (may Allah have mercy on him) said: Rather what is meant by wudoo’ here, in our opinion, is washing off blood and whatever else had gotten onto the body, not wudoo’ for prayer. It was narrated from Ibn Mas‘ood that he washed his hands after eating, then he wiped his face with his wet hands and said: This is the wudoo’ of one who has not broken his wudoo’. This (usage of the word wudoo’) is well known among the Arabs, who use the word to refer to washing some part of the body, not all the parts that should be washed in the case of wudoo’ for prayer.

End quote from as-Sunan al-Kubra, 1/143 

Further evidence in support of the majority view is the report narrated by Abu Dawood (205) from ‘Ali ibn Talq (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “If one of you breaks wind whilst praying, let him leave and do wudoo’, then repeat the prayer.”

There is a difference of opinion as to the soundness of this hadeeth. It was classed as saheeh by Ibn Hibbaan and as hasan by others. 

In Fataawa al-Lajnah ad-Daa’imah there is a question about the hadeeth, “Whoever breaks wind during the prayer, let him exit the prayer. If that is during a congregational prayer, let him hold his nose and leave, and do wudoo’, then let him resume his prayer from where he left off, so long as he does not speak.” 

They replied: This hadeeth was classed as da‘eef by some of the leading scholars of hadeeth. Hence what it indicates about the one who breaks his wudoo’ during the prayer exiting the prayer, then doing wudoo’ and coming back to complete what is left of his prayer, is not correct. Rather breaking wudoo’ during the prayer renders it invalid, and after doing wudoo’ the individual has to start the prayer all over again from the beginning, as is indicated by the hadeeth of ‘Ali ibn Talq (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “If one of you breaks wind whilst praying, let him leave and do wudoo’, then repeat the prayer.” Narrated by Ahmad, Abu Dawood, at-Tirmidhi, an-Nasaa’i and Ibn Maajah; classed as saheeh by Ibn Hibbaan.

End quote from Fataawa al-Lajnah ad-Daa’imah, 5/438, vol. 2 

Shaykh Ibn Baaz (may Allah have mercy on him) said: If a person loses his wudoo’ whilst praying, by breaking wind or having a severe nosebleed and so on, then his prayer is rendered invalid, according to the more correct of the two scholarly opinions, because the Prophet (blessings and peace of Allah be upon him) said: “If one of you breaks wind whilst praying, let him leave and do wudoo’, then repeat the prayer.” Narrated by Imam Ahmad and the authors of as-Sunan. This was also mentioned by al-Haafiz Ibn Hajar in al-Buloogh. 

With regard to the hadeeth which mentions resuming the prayer from where one left off, this is a da‘eef hadeeth, as was also explained by al-Haafiz Ibn Hajar in al-Buloogh.

End quote from Fataawa ash-Shaykh Ibn Baaz, 10/159 

Secondly: 

It is preferable for the one who is praying behind an imam not to say the tasleem after the prayer until the imam has finished the second tasleem. See the answer to question no. 75977. 

And Allah knows best.