Musnad Ahmad 1118:34

Qutaybah ibn Sa'id narrated to us, he said: Layth ibn Sa'd narrated to us, from 'Ubayd Allah ibn Abi Ja'far, from al-Julah Abu Kathir, he said: Hanash al-San'ani narrated to me, from Fadalah ibn 'Ubayd, he said: We were with the Messenger of Allah, may Allah bless him and grant him peace, on the day of Khaybar, trading with the Jews an *uqiyah* of gold for two or three dinars. The Messenger of Allah, may Allah bless him and grant him peace, said: "Do not sell gold for gold unless it is weight for weight."