Fiqh-us-Sunnah Volume 002, Supererogatory Prayer, Fiqh 2.034.

Section : Salatul Taubah, the prayer of penitence.

Abu Bakr reports: "I heard the Prophet sallallahu alehi wasallam saying: 'Allah forgives the man who commits a sin (then feels ashamed), purifies himself, offers a prayer and seeks His forgiveness.' Then he recited the 'ayyah: 'And those who, when they do an evil thing or wrong themselves, remember Allah and implore forgiveness for their sins - and who can forgive sins except Allah? - and will not knowingly repeat (the wrong) they did. The reward of such will be forgiveness from their Lord, and gardens underneath which rivers flow, wherein they will abide forever- a bountiful reward for workers."' [al-'lmran: 135-136]. This is related by Abu Dawud, an-Nasa'i, Ibn Majah, al-Baihaqi, and at-Tirmizhi who calls it hasan.

At-Tabarani records in al-Mu'jam al-Kabir, with a hasan chain, from Abu ad-Darda' that the Prophet sallallahu alehi wasallam said: "Whoever makes wudu' and perfects the wudu' and then stands and prays two rak'at or four rak'at, obligatory or non-obligatory, and perfects therein his ruku' and sujjud and then asks for Allah's forgiveness, he will be forgiven."