Musnad Abu Ya'la al-Mawsili 152:13

Muhammad ibn al-Sabah narrated to us, Sharik narrated to us, from Abd al-Karim, from Ziyad ibn al-Jarrah, from Ibn Ma'qil, who said: I entered with my father upon Abdullah ibn Mas'ud, and I heard him say: The Messenger of Allah, may Allah bless him and grant him peace, said: "Regret is repentance."