Musannaf Ibn Abi Shayba 13:2402

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: al-Mas’udi narrated to us, from al-Qasim ibn ‘Abd al-Rahman, who said: A dispute was brought before Shurayh regarding a sheep that eats flies. He said: "Its milk is pure and its fodder is free," so he permitted it.