Musnad Ahmad 717:1

'Abd al-Samad narrated to us, my father narrated to me, Husayn—meaning al-Mu'allim—narrated to us, from Ibn Buraydah, Hanzalah ibn 'Ali narrated to me, that Mihjan ibn al-Adra' narrated to him, that the Messenger of Allah (may Allah bless him and grant him peace) entered the mosque, and there was a man who had finished his prayer and was reciting the tashahhud, and he was saying: "O Allah, I ask You by Allah, the One, the Unique, the Eternal Refuge, who neither begets nor is born, and to whom there is no equivalent, that You forgive me my sins, for indeed You are the Forgiving, the Merciful." He said: The Prophet of Allah (may Allah bless him and grant him peace) said: "He has been forgiven, he has been forgiven, he has been forgiven," three times.