Musnad Abu Ya'la al-Mawsili 6:24

Suwayd ibn Sa'id narrated to us, from Malik, from 'Abdullah ibn Yazid, the freed slave of al-Aswad, that Zayd Abu 'Ayyash informed him that he asked Sa'd about the white [grain], meaning the *salt* (a type of barley). Sa'd said, "Which of the two is better?" He said, "The white one." So he forbade him from that. And Sa'd said, "I heard the Messenger of Allah (may Allah bless him and grant him peace) being asked about buying dry dates with fresh dates. The Messenger of Allah (may Allah bless him and grant him peace) said, 'Do fresh dates decrease when they dry?' They said, 'Yes.' So he forbade that."