Sunan Abi Dawud — General Behavior (Kitab Al-Adab) — Hadith No: 5047
Translation
Al-Bara b. Azib said :The Messenger of Allah (May peace be upon him) said to me: when you go to bed while you are in the state of purification, lay your head on your right hand. He then mentioned the rest of the tradition in a similar manner as above