Musnad Ahmad 26:462

Kathir ibn Hisham narrated to us, he said: Furat narrated to us, from Abd al-Karim, from Ziyad ibn al-Jarrah, from Abdullah ibn Ma'qil, he said: My father was with Abdullah 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."