Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1453
Abu Qatada reported that when the Messenger of Allah (may peace be upon him) was in a journey he got down for rest at night, and he used to lie down on his right side, and when he lay down for rest before the dawn, he used to stretch his forearm and place his head over his palm.