Musannaf Ibn Abi Shayba 33:773

Waki’ narrated to us, from Sufyan, from Mansur, from Mujahid, from ‘Ubayd ibn ‘Umayr regarding the saying: "To every oft-returning (to Allah), keeper (of His covenant)." He said: "He is the one who remembers his sins in private, then seeks forgiveness for them."