Musannaf Ibn Abi Shayba 33:1367

Shadhan narrated to us, he said: Mahdi ibn Maymun narrated to us, from Yunus ibn Khabbab, he said: Mujahid said to me, "Shall I not inform you of the 'Owwab' (the oft-returning) who is 'Hafiz' (the keeper/guardian)?" I said, "Yes." He said, "He is the one who remembers his sin when he is alone, so he seeks forgiveness from Allah for it."