Musannaf Ibn Abi Shayba 17:1227

Marwan ibn Mu'awiyah narrated to us, from Abu Malik al-Ashja'i, from 'Abd al-Rahman ibn Nawfal al-Ashja'i, from his father, who said: I said, "O Messenger of Allah, inform me of something I should say when I reach the morning and when I reach the evening." He said, "Recite 'Say, O disbelievers,' then sleep upon its conclusion, for it is an acquittal from polytheism."