Musnad Ahmad 31:457

Waki’ narrated to us, Sulayman ibn Ali al-Rab’i narrated to us, he said: I heard Abu al-Jawza’ say: I heard Ibn Abbas giving a legal opinion (fatwa) regarding currency exchange (sarf). He said: So I gave that opinion for a time. He said: Then I met him, and he retracted it. He said: So I said to him: "And why?" He said: "It was only an opinion I held. Abu Sa’id al-Khudri narrated to me that the Messenger of Allah, may Allah bless him and grant him peace, forbade it."