Musnad Ahmad 26:21

Sufyan narrated to us, from Abd al-Karim, who said: Ziyad ibn Abi Maryam informed me, from Abdullah ibn Ma'qil ibn Muqarrin, who said: I entered with my father upon Abdullah ibn Mas'ud, and he said: "Did you hear the Prophet, may Allah bless him and grant him peace, say: 'Remorse is repentance'?" He said: "Yes." And he said once: "I heard him say: 'Remorse is repentance.'"