Allaah says (interpretation of the meaning): 

“I said (to them): ‘Ask forgiveness from your Lord, verily, He is Oft‑Forgiving;

11. ‘He will send rain to you in abundance,

12. ‘And give you increase in wealth and children, and bestow on you gardens and bestow on you rivers.’”

[Nooh 71:10-12] 

Al-Qurtubi said: 

The words “I said (to them): ‘Ask forgiveness from your Lord’” mean: ask Him for forgiveness of your previous sins with sincerity of faith.

“verily, He is Oft‑Forgiving” means, He forgives everyone who turns to Him; this encourages us to repent.

“He will send rain to you in abundance” means, He will send a great deal of rain to you.

“And give you increase in wealth and children, and bestow on you gardens and bestow on you rivers”  

Al-Shu’bi said: ‘Umar went out to pray for rain and he did no more than pray for forgiveness until he came back, then it rained. They said: “We did not see you ask for rain.” He said: “I sought rain with the real key by means of which rain is sought.” Then he recited (interpretation of the meaning): “Ask forgiveness from your Lord, verily, He is Oft‑Forgiving; He will send rain to you in abundance.” 

The word translated here as “key” – majaadeeh – refers to the stars which the ancient Arabs claimed brought rain. What ‘Umar (may Allaah be pleased with him) meant was to reject this false claim of the Arabs and explain that he had sought rain by the correct means, which is praying for forgiveness, not by means of the stars. 

A man complained to al-Hasan about a drought, and he said to him: “Pray to Allaah for forgiveness.” Another man complained to him of poverty and he said to him: “Pray to Allaah to forgive you.” Another man said to him: “Pray to Allaah to bless me with a child.” He said: “Pray to Allaah for forgiveness.” Another complained to him that his garden was dry. He said to him: “Pray to Allaah for forgiveness.” We asked him about that and he said: “This is not my personal opinion, for Allaah says in Soorat Nooh (interpretation of the meaning): ‘Ask forgiveness from your Lord, verily, He is Oft‑Forgiving; He will send rain to you in abundance. And give you increase in wealth and children, and bestow on you gardens and bestow on you rivers.” 

Tafseer al-Qurtubi, 18/301-302. 

Secondly: 

With regard to the wording of prayers for forgiveness, it is better to use the words narrated in the saheeh Sunnah from the Prophet (peace and blessings of Allaah be upon him), which he either said himself or enjoined the ummah to say: 

(i)                It was narrated from Shaddaad ibn Aws (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “The best prayer for forgiveness is to say: Allaahumma anta Rabbiy laa ilaaha illa anta, khalaqtani wa ana ‘abduka wa ana ‘ala ‘ahdika wa wa’dika ma astata’tu, a’oodhu bika min sharri ma sana’tu aboo’u laka bi ni’matika ‘alayya wa aboo’u laka bi dhanbi, faghfir li fa innahu laa yaghfir ul-dhunooba illa anta (O Allaah, You are my Lord and I am Your slave, You have created me and I am faithful to my covenant and my promise (to You) as much as I am able. I seek refuge with You from the evil of that which I have done. I acknowledge before You all the blessings You have bestowed upon me and I confess to You my sin. Forgive me for there is no one who forgives sin except You).”  

He said: Whoever says this during the day believing in it with certainty and dies that day before evening comes, will be one of the people of Paradise, and whoever says it at night believing in it with certainty and dies before morning comes will be one of the people of Paradise. Narrated by al-Bukhaari, 5947. 

(ii)              It was narrated from Abu Moosa al-Ash’ari that the Prophet (peace and blessings of Allaah be upon him) used to recite the following du’aa’: “Rabb ighfir li khati’ati wa jahli wa israafi fi amri kullihi wa ma anta a’lam bihi minni, Allaahumma ighfir li khataayaaya wa ‘amdi wa jahli wa hazli wa kulla dhaalika ‘indi, Allaahumma ighfir li ma qaddamtu wa ma akhkhartu wa ma asrartu wa ma a’lantu anta al-Muqaddim wa anta al-Mu’akhkhir wa anta ‘ala kulli shay’in qadeer (O Allaah, forgive me my mistakes, my ignorance and my transgressing the limits of righteousness in my deeds and whatever You know better than I. O Allaah, forgive me the wrongs that I have committed deliberately or mistakenly or jokingly, for all of that is possible in me. O Allaah, forgive my past and future sins, what I have done secretly and openly, for You are the One Who brings (some people) forward and puts (others) back, and You are able to do all things).” Narrated by al-Bukhaari (6035), Muslim (2719). 

(iii)            It was narrated that Ibn ‘Umar said: We used to count that the Messenger of Allaah (S) said one hundred times in a gathering: “Rabb ighfir li wa tub ‘alayya innaka anta al-tawwaab ul-raheem (O Allaah forgive me and accept my repentance, for You are the Accepter of repentance, the Most Merciful). 

Narrated by al-Tirmidhi, 3434, where it says al-Tawwaab al-Ghafoor (the Accepter of repentance, the Oft-Forgiving); Abu Dawood, 1516; Ibn Maajah, 3814. 

(iv)            It was narrated from Abu Yasaar that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever says Astaghfir Allaah al-‘Azeem alladhi laa ilaaha illa huwa al-Hayyu al-Qayyoom wa atoobu ilayhi (I ask forgiveness of Allaah the Almighty, besides Whom there is no god, the Ever-Living, the Eternal, and I repent to Him), will be forgiven even if his sin is fleeing from the battlefield [a major sin].” 

Narrated by al-Tirmidhi, 3577; Abu Dawood, 1517. 

(v)              It was narrated from Abu Bakr al-Siddeeq (may Allaah be pleased with him) that he said to the Messenger of Allaah (peace and blessings of Allaah be upon him): “Teach me a du’aa’ which I may recite in my prayer.” He said: “Say: Allaahumma inni zalamtu nafsi zulman katheeran wa laa yaghfir ul-dhunooba illa anta faghfir li maghfiratan min ‘indaka warhamni innaka anta al-Ghafoor al-Raheem (O Allaah, I have wronged myself greatly and no one forgives sins but You, so grant me forgiveness from You and have mercy on me, for You are the Oft-Forgiving, Most Merciful).” 

Narrated by al-Bukhaari, 799; Muslim, 2705. 

And Allaah knows best.