Musnad Ahmad 26:464

Mu'ammar ibn Sulayman al-Raqqi narrated to us, he said: Khusayf narrated to us, from Ziyad ibn Abi Maryam, from 'Abdullah ibn Mu'aqqil, he said: My father was with Ibn Mas'ud, and he heard him saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Regret is repentance."