Musnad Ahmad 30:693

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from 'Ubayd Allah ibn 'Umar, from Sa'id ibn Abi Sa'id al-Maqburi, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you rises from the night, then returns to his bed, let him shake off his bed with the inner part of his lower garment, for he does not know what he left behind it after. Then let him say: 'In Your name, O Allah, I have placed my side, and in Your name I lift it. O Allah, if You withhold my soul, then forgive it, and if You send it back, then protect it with that which You protect Your righteous servants.'"