Musannaf Ibn Abi Shayba 13:2360

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Hisham narrated to us, from Qatadah, from Abu Hassan al-A’raj, from Ibn Abbas, who said: I bear witness that the *salaf* (forward sale) for a specified term is something that Allah has made lawful and permitted. Then he recited: "O you who have believed, when you contract a debt for a specified term, write it down."