Musannaf Ibn Abi Shayba 13:888

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Simak, from Ikrimah, from Ibn Abbas, he said: "If you have entered into a forward sale (salam) for food, then do not take in its place other food. And if you wish to take in its place fodder, then take it if you wish."