Fiqh-us-Sunnah, Volume 2: Salatul Taubah, the prayer of penitence
Abu Bakr reports: "I heard the Prophet sallallahu alehi wasallamsaying: 'Allah forgives the man who commits a sin (then feels ashamed),purifies himself, offers a prayer and seeks His forgiveness.' Then he recitedthe 'ayyah: 'And those who, when they do an evil thing or wrong themselves,remember Allah and implore forgiveness for their sins - and who can forgivesins except Allah? - and will not knowingly repeat (the wrong) they did. Thereward of such will be forgiveness from their Lord, and gardens underneathwhich rivers flow, wherein they will abide forever- a bountiful reward forworkers."' [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 Abuad-Darda' that the Prophet sallallahu alehi wasallam said: "Whoever makeswudu' and perfects the wudu' and then stands and prays two rak'at or fourrak'at, obligatory or non-obligatory, and perfects therein his ruku' and sujjudand then asks for Allah's forgiveness, he will be forgiven."