Musnad Ahmad 27:269

Yahya ibn Sa'id narrated to us, 'Ubaydullah narrated to us, Nafi' informed me, from 'Abdullah ibn 'Umar, who said: "They used to trade food in bulk at the upper part of the market, so the Messenger of Allah (peace and blessings of Allah be upon him) forbade them from selling it until they had moved it."