sahih bukhari(247) Hadith Number 6.

Abu Qataadah Radiyallahu 'Anhu relates: "If Rasulullah Sallallahu 'Alayhi Wasallam made an early joumey-break in the last portion of the night on his travels, he slept on his right side. If he was staying there till before the morning, he lifted his right arm, put his head on it and slept".