Musnad Ahmad 1007:116

'Abd al-Samad narrated to us, Hammad narrated to us, Humayd narrated to us, from Bakr, from 'Abd Allah ibn Rabah, from Abu Qatadah, that the Messenger of Allah (peace and blessings of Allah be upon him) used to: "When he would halt for the night, he would lie on his right side. And when he would halt shortly before dawn, he would raise his forearms and place his head between his palms."