It is narrated from Sayyiduna Mihjan bin al-Adra' (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) entered the mosque, and there was a man who had completed his prayer and was sitting in the tashahhud, saying: O Allah, I ask You, by Allah, the One, the Self-Sufficient, who did not beget and was not begotten, and to whom no one is equal, that You forgive me my sins; indeed You are the Most Forgiving, the Most Merciful. So the Messenger of Allah (peace and blessings be upon him) said: "He has been forgiven, he has been forgiven, he has been forgiven."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17834