Sunan An-Nasai, Chapter: The Book of Purification

"I came to a man called Safwan bin Assal and sat at his door. He came out and said: 'What do you want?' I said: 'I am seeking knowledge.' He said: 'The angels lower their wings for the seeker of knowledge out of pleasure at what he is seeking.' He said: 'What do you want to know about?' I said: About the Khuffs.' He said: 'When we were with the Messenger of Allah (saw) on a journey, he told us not to take them off for three days except in the case of Janabah, but not in the case of defecation, urinating or sleep.'" (Hasan)