Musannaf Ibn Abi Shayba 21:118

Ibn Fudayl narrated to us, from Abd al-Rahman ibn Ishaq, from al-Qasim ibn Abd al-Rahman, from Abdullah ibn Mas’ud, that he said: Whoever wakes up during the night and says: "There is no god but You, my Lord; I have wronged my soul, so forgive me," he will emerge from his sins just as a snake emerges from its slough.