Musannaf Ibn Abi Shayba 13:1370

Abu Bakr narrated to us, he said: Ali bin Mushir and Ibn Abi Za'idah both narrated to us, from Ubaydullah, from Nafi', from Ibn Umar, who said: The Messenger of Allah (peace be upon him) said: "If one of you buys food, he shall not sell it until he takes possession of it." Ibn Abi Za'idah said: "And he takes possession of it."