Musnad Ahmad 26:574

Waki’ and ‘Abd al-Rahman narrated to us—the meaning is the same, and this is the wording of Waki’—Sufyan narrated to us, from ‘Abd al-Karim al-Jazari, from Ziyad ibn Abi Maryam, from ‘Abd Allah ibn Mu’qal, that his father, Mu’qal ibn Muqarrin al-Muzani, said to Ibn Mas’ud: "Did you hear the Messenger of Allah, may Allah bless him and grant him peace, say: 'Remorse is repentance'?" He said: "Yes."