Musnad Ahmad 31:489

Yazid ibn Harun narrated to us, Sulayman ibn Ali al-Rab'i informed us, Abu al-Jawza' narrated to us more than once, he said: I asked Ibn Abbas about currency exchange (al-sarf) hand-to-hand. He said: "There is no harm in that, two for one, more than that, or less." He said: Then I performed Hajj another time, and the Sheikh was still alive, so I came to him and asked him about currency exchange. He said: "Weight for weight." He said: I said, "You had given me a fatwa of two for one, and I have continued to give fatwa by it since you gave it to me." He said: "That was based on my own opinion, and this is Abu Sa'id al-Khudri narrating from the Messenger of Allah, may Allah bless him and grant him peace." So I abandoned my opinion for the hadith of the Messenger of Allah, may Allah bless him and grant him peace.