مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3868
Musnad of Abdullah bin Mas'ood
It was narrated that Abu Ishaq said: I heard Abu Ubaidah narrate that his father said: The Prophet (ﷺ) often used to say: "O Allah, glory and praise be to You; O Allah, forgive me.” When the soorah "When there comes the Help of Allah (to you, O Muhammad (ﷺ) against your enemies) and the Conquest (of Makkah)" (an-Nasr 110:11) was revealed, he started saying: "O Allah, glory and praise be to You, O Allah, forgive me, for You are the Acceptor of repentance."
Musnad Ahmad 3891Hasan because of corroborating evidence, its isnad is da'eef