Musnad Ahmad 31:490

Yazid ibn Harun narrated to us, Ibn ‘Awn informed us, from Nafi’, who said: A man was narrating to Ibn ‘Umar a hadith from Abu Sa’id al-Khudri regarding *sarf* (currency exchange). He said: Then Abu Sa’id arrived and stayed in this house. Ibn ‘Umar took my hand and the hand of the man until we came to Abu Sa’id. He stood over him and said: "What is this that this man is narrating from you?" Abu Sa’id said: "Yes, my eyes saw it and my ears heard it." And he pointed with his fingers to his eyes and his ears. "I have not forgotten what he said with his two fingers from the Messenger of Allah, peace and blessings of Allah be upon him: 'He forbade gold for gold, and silver for silver, except equal for equal, like for like. Beware, do not sell what is absent for what is present, and do not increase one of them over the other.'"