Musnad Ahmad 30:2245

'Affan narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, Sulayman al-A'mash narrated to us, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you prays the two rak'ahs before the morning prayer, let him lie down on his right side."