Musnad Ahmad 31:504

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, Nafi' informed me, he said: It reached Ibn 'Umar that Abu Sa'id al-Khudri was narrating a hadith from the Prophet, may Allah bless him and grant him peace, regarding currency exchange (sarf). So he took my hand, and I went with him and the man. He said: "What is this hadith that has reached me from you, which you narrate from the Prophet, may Allah bless him and grant him peace, regarding currency exchange?" He replied: "My ears heard it and my heart retained it from the Messenger of Allah, may Allah bless him and grant him peace, saying: 'Do not sell gold for gold, except like for like, and do not sell silver for silver, except like for like, and do not prefer some of it over others, and do not sell from it that which is absent for that which is present.'"