Musannaf Ibn Abi Shayba 13:1380

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Ibn Tawus, from his father, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever buys food, let him not sell it until he has measured it." I said to Ibn Abbas: "Why?" He said: "Do you not see that they buy gold while the food is deferred?"