Musannaf Ibn Abi Shayba 13:1740

Abu Bakr narrated to us, he said: Sahl ibn Yusuf narrated to us, from Humayd, from Abu Nadra, who said: I said to Ibn Umar: "Our commanders forbid us from it," meaning the *salam* (forward sale) of animals and servants. He said: "Obey your commanders if they forbid it." And their commanders in those days were like al-Hakam al-Ghafari and Abd al-Rahman ibn Samura.