Musannaf Ibn Abi Shayba 13:891

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mansur, he said: Abd al-Salam narrated to us, from Abu Hamzah, from Ibrahim, he said: Umar said: "If you enter into a salam (forward sale) contract for something, then do not sell it until you take possession of it, and do not exchange it for something else."