مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4322
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah bin Mas'ood (رضي الله عنه) said: After the verse "When there comes the Help of Allah (to you, O Muhammad (ﷺ) against your enemies) and the Conquest (of Makkah)" (an-Nasr 110:1) was revealed, the Prophet (ﷺ) would often say: “Glory and praise be to You, O Allah. O Allah, forgive me, for You are the Acceptor of repentance."
Musnad Ahmad 4352Hasan because of corroborating evidence; its isnad is da'eef