Musnad Ahmad 8:105

'Abd al-Rahman ibn Mahdi narrated to us, from Malik, from 'Abdullah ibn Yazid, from Zayd ibn 'Ayyash, who said: Sa'd was asked about [the exchange of] white wheat for barley, and he disliked it and said: "I heard the Prophet, peace and blessings of Allah be upon him, being asked about [the exchange of] fresh dates for dried dates. He said: 'Do they decrease when they dry?' They said: 'Yes.' He said: 'Then do not [do it].'"