Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 980
Narated By Mihjan ibn al-Adra' : The Apostle of Allah (pbuh) entered the mosque and saw a man who had finished his prayer, and was reciting the tashahhud saying: O Allah, I ask you, O Allah, the One, the eternally besought of all, He begetteth not, nor was He begotten, and there is none comparable unto Him, that you may forgive me my sins, you are Most Forgiving, Merciful.
He (the Prophet) said: He was forgiven (repeating three times.)