Musnad Ahmad 8:113

Sufyan narrated to us, from Isma'il bin Umayyah, from 'Abdullah bin Yazid, from Abu 'Ayyash, who said: Sa'd was asked about selling *salt* (a type of barley) for barley—or something similar to this—so he said: The Prophet, peace and blessings of Allah be upon him, was asked about [selling] dry dates for fresh dates. He said: "Does the fresh date decrease when it dries?" They said: Yes. He said: "Then do not [do it]."