Musannaf Ibn Abi Shayba 18:1028

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Sufyan al-Thawri narrated to us, from 'Abd al-Karim, from Ziyad ibn Abi Maryam, from 'Abd Allah ibn Mu'aqqil, that his father, Mu'aqqil ibn Muqarrin al-Muzani, said to Ibn Mas'ud: "Did you hear the Prophet, may Allah bless him and grant him peace, say: 'Repentance is remorse'?" He said: "Yes."