Musnad Ahmad 35:6

Yahya ibn Sa'id narrated to us, Hisham—meaning al-Dastawa'i—narrated to us, Yahya ibn Abi Kathir narrated to me, from a man, that Yusuf ibn Mahak informed him, that 'Abdullah ibn 'Ismah informed him, that Hakim ibn Hizam informed him, he said: I said: "O Messenger of Allah, I engage in sales, so what is lawful for me from them, and what is unlawful for me?" He said: "If you buy a sale, do not sell it until you take possession of it."