Musannaf Ibn Abi Shayba 16:638

Abu Bakr narrated to us, he said: Abd al-Samad ibn Abd al-Warith narrated to us, from Hammad ibn Salamah, from Thabit, from Anas, who said: When Abu Musa would sleep, he would wear a *tubban* (drawers/shorts) for fear that his *awrah* (private parts) might become exposed.