Musnad Ahmad 26:466

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