Musannaf Ibn Abi Shayba 13:1208

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Shaqiq ibn Salamah, Abu Wa’il, narrated to us, he said: I heard Ibn ‘Umar being asked about a man who had a debt owed to him by another man, and he wanted to enter into a *salam* (forward sale) contract with him for food. He disliked that and said: "No, not until he takes possession of it."