Allah, the Exalted, says:
“And those who came after them say: “Our Lord! Forgive us, and our brethren who came before us into the Faith, and leave not, in our hearts, rancour (or sense of injury) against those who have believed. Our Lord! Thou art indeed Full of Kindness, Most Merciful.”” (Surah Al Hashr 59:10)
“Know, therefore, that there is no god but Allah, and ask forgiveness for thy fault, and for the men and women who believe: for Allah knows how ye move about and how ye dwell in your homes”. (Surah Muhammad 47:19)
“O our Lord! cover (us) with Thy Forgiveness – me, my parents, and (all) Believers, on the Day that the Reckoning will be established!” (Surah Ibrahim 14:41)
Abu Dharr reported that Allah’s Messenger (may peace be upon him) said: “There is no believing servant who supplicates for his brother behind his back (in his absence) that the Angels do not say: ‘The same be for you too.'” (Sahih Muslim, Book #035, Hadith #6588)
Commentary: This Hadith makes it evident that one who prays for someone in his absence also stands to benefit from it because the angel appointed for the task of praying for those who pray for others will pray for him, saying, “O Allah! Grant him also the same which he has asked for others.”
Umm Darda’ reported: “My husband reported that he heard Allah’s Messenger (may peace be upon him) as saying: He who supplicates for his brother behind his back (in his absence), the Angel commissioned (for carrying supplication to his Lord) says: ‘Ameen, and it is for you also.'” (Sahih Muslim, Book #035, Hadith #6589)
Safwan (and he was Ibn ‘Abdullah b. Safwan, and he had been married to Umm Darda’) reported: “I visited Abu Darda’s house in Syria. I did not find him there but Umm Darda’ (was present at the house). She said: Do you intend to perform Hajj during this year? I said: Yes. She said: Do supplicate Allah for blessings upon us, for Allah’s Apostle (may peace be upon him) used to say: The supplication of a Muslim for his brother at his back (in his absence) is responded so long as he makes a supplication for blessings for his brother and the commissioned angel says: ‘Ameen, and says: May it be for you too’. I went to the bazar and met Abfi Dardi’ and he narrated like this from Allah’s Messenger (may peace be upon him).” (Sahih Muslim, Book #035, Hadith #6590)