Musnad Ahmad 769:21

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, and Hajjaj narrated to me, Shu'bah said: I heard Abdullah ibn Abi al-Mujalid say: Abdullah ibn Shaddad and Abu Burdah differed regarding the *salam* (forward sale). They sent me to Abdullah ibn Abi Awfa, and I asked him. He said: "We used to engage in *salam* during the time of the Messenger of Allah, peace and blessings of Allah be upon him, and Abu Bakr and Umar, may Allah the Exalted be pleased with them, in wheat, barley, raisins, or dates"—he was in doubt regarding dates and raisins—"and what they had with them, or what we saw with them." Then I went to Abd al-Rahman ibn Abza, and he said the same.