Musannaf Ibn Abi Shayba 28:814

Isma'il ibn 'Ayyash narrated to us, from Yahya ibn Abi 'Amr al-Shaybani, from 'Abdullah ibn Muhayriz, Khalid ibn al-Durayk, and others, that they used to say regarding a man who finds food and fodder in the land of the Byzantines: They said: He may eat, feed others, and provide it as fodder. But if he sells any of that for gold or silver, he must return it to the spoils of the Muslims.