Musnad Abu Ya'la al-Mawsili 159:19

Abu Khaythama narrated to us, Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, who said: Do not meet the riders [bringing goods] for sale, and do not tie up the udders of camels and sheep. Whoever buys them after that, he is at the best of the two options: if he wishes, he may keep them, and if he wishes, he may return them along with a sa' of dates, not wheat. Abu Khaythama said: He means wheat.