Musannaf Ibn Abi Shayba 13:2690

Abu Bakr narrated to us, he said: Muhammad ibn Bishr narrated to us, from Ibn Jurayj, from Abdullah ibn Tawus, from his father, who said: "Food should not be exchanged for food in a forward sale (salam), nor food for meat." And he did not see any harm in exchanging food for a standing sheep in a forward sale.