Maliks Muwatta Book 31, Hadith Number 22.

Yahya related to me from Malik from Abdullah ibn Yazid that Zayd ibn Ayyash told him that he had once asked Sad ibn Abi Waqqas about selling white wheat for a type of good barley. Sad asked him which was the better and when he told him the white wheat, he forbade the transaction. Sad said, “I heard the Messenger of Allah, may Allah bless him and grant him peace, being asked about selling dried dates for fresh dates, and the Messenger of Allah, may Allah bless him and grant him peace, said, ‘Do the dates diminish in size when they become dry?’ When he was told that they did, he forbade that.”