Musnad Ahmad 830:123

Isma'il narrated to us, Yahya ibn Abi Ishaq narrated to me, 'Abd al-Rahman ibn Abi Bakrah narrated to us, he said: Abu Bakrah said: "The Messenger of Allah (may Allah bless him and grant him peace) forbade us from buying silver with silver, and gold with gold, unless it is equal for equal. And he commanded us to buy silver with gold, and gold with silver, however we wish." Thabit ibn 'Abd Allah said to him: "Hand to hand?" He replied: "That is how I heard it."