The Book of Prayers (Kitab Al-Salat)

Muslim :: Book 4 : Hadith 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.

Source materials are from the University of Southern California MSA site
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