Musannaf Ibn Abi Shayba 35:197

Waki’ narrated to us, from Malik ibn Anas, from ‘Abdullah ibn Yazid, from Zayd Abu ‘Ayyash, who said: I asked Sa’d about [trading] naked barley for millet, and he disliked it. And Sa’d said: The Prophet, may Allah bless him and grant him peace, was asked about [trading] fresh dates for dried dates. He said, "Do they decrease when they dry?" We said, "Yes." He said, "Then he forbade it."