Musannaf Ibn Abi Shayba 17:1233

Abu Bakr narrated to us, he said: Husayn bin Ali narrated to us, from Za'idah, from 'Asim, from al-Musayyab, from Hafsah, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would take to his bed, he would place his right hand under his right cheek.