Musannaf Ibn Abi Shayba 13:2555

Abu Bakr narrated to us, he said: Isma'il ibn Ibrahim narrated to us, from Habib ibn Shahid, he said: Layl al-'Uqayli came to Ibn Sirin, and with him was a man. He said: "Indeed, this man is asking you about currency exchange (al-sarf)." He said: "The Prophet, may Allah bless him and grant him peace, forbade it, as did Abu Bakr, Umar, and Uthman."