Musannaf Ibn Abi Shayba 17:1237

26539. 'Abdullah ibn Numayr narrated to us, he said: 'Ubaydullah ibn Isma'il narrated to us, from Sa'id ibn Abi Sa'id al-Maqburi, from Abu Hurayrah, that the Prophet, may Allah bless him and grant him peace, said: "When one of you intends to lie down on his bed, let him lie down on his right side."