Musannaf Ibn Abi Shayba 21:183

Abdullah bin Numayr narrated to us, Ubaydullah bin Umar narrated to us, from Sa'id bin Abi Sa'id al-Maqburi, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you intends to lie down on his bed, let him take the inner part of his lower garment and dust his bed with it, for he does not know what he left behind upon it. 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. So if You take my soul, have mercy on it, and if You send it back, protect it with that which You protect Your righteous servants.'"