Musannaf Ibn Abi Shayba 13:650

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Shu’bah, from Yaman Abu Hudhayfah, from Ziyad, the freed slave of Ibn Abbas, that Ibn Abbas was asked about a man who mixes barley with wheat and then sells it. He said: There is no harm in it.