Musannaf Ibn Abi Shayba 27:487

'Abdullah ibn Idris narrated to us, from Layth, from Mujahid, who said: 'Ali said: "No one acted upon it before me, and no one will act upon it after me. I had a dinar, so I sold it for ten dirhams. Whenever I would hold a private conversation with the Messenger of Allah (may Allah bless him and grant him peace), I would give a dirham in charity until they were exhausted." Then he recited this verse: "O you who have believed, when you [wish to] privately consult the Messenger, present before your consultation a charity."