Musannaf Ibn Abi Shayba 13:2873

Abu Bakr narrated to us, he said: Hafs narrated to us, from Hisham, from al-Hasan, who said: The Messenger of Allah (peace be upon him) forbade the selling of food until the two measures (sa') flow in it, so its increase belongs to the one who bought it, and its decrease is upon the seller.