Musnad Ahmad 30:2342

Yahya ibn Sa'id al-Umawi narrated to us, he said: 'Ubayd Allah—meaning Ibn 'Umar ibn Hafs ibn 'Asim—narrated to us, from Sa'id ibn Abi Sa'id, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you comes to his bed, let him take the inner part of his lower garment and dust off his bed with it, for he does not know what occurred upon it after him. Then let him lie on his right side, then let him say: 'In Your name, my Lord, I have placed my side, and by You I lift it. If You withhold my soul, then have mercy upon it, and if You send it forth, then protect it with that which You protect Your righteous servants.'"