Musnad Ahmad 1155:39

Husayn ibn Ali narrated to us, from Za'idah, from 'Asim, from al-Musayyib, from Hafsa, the wife of the Prophet, may Allah bless him and grant him peace, who said: "When the Messenger of Allah, may Allah bless him and grant him peace, would take to his bed, he would place his right hand under his right cheek. His right hand was for his food, his purification, his prayer, and his clothing, while his left hand was for everything other than that. And he used to fast on Mondays and Thursdays."