Musnad Abu Ya'la al-Mawsili 152:61

Abu Khaythama narrated to us, Ibn 'Uyaynah narrated to us, 'Abd al-Karim al-Jazari narrated to us, from Ziyad ibn Abi Maryam, from 'Abd Allah ibn Ma'qal, who said: I entered with my father upon 'Abd Allah ibn Mas'ud. He said: "Did you hear the Messenger of Allah, may Allah bless him and grant him peace, say: 'Remorse is repentance'?" He said: "Yes."