مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3987
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah bin Maʼqil said: My father was with ‘Abdullah bin Mas'ood and heard him say: I heard The Messenger of Allah (ﷺ) say: "Regret is repentance.”
Musnad Ahmad 4012A saheeh hadeeth, this is a qawi isnad]