Musnad Ahmad 31:467

'Abd al-Malik ibn 'Amr narrated to us, Hisham and Yazid narrated to us, Hisham informed us, from Yahya ibn Abi Kathir, from Abu Salamah, from Abu Sa'id, who said: "We used to be provided with 'jam' dates" — Yazid said: "dates from the 'jam' dates" — "during the time of the Messenger of Allah (peace and blessings of Allah be upon him), and we would sell two sa's for one sa'. This reached the Prophet (peace and blessings of Allah be upon him), so he said: 'Do not [sell] two sa's of dates for one sa', nor two sa's of wheat for one sa', nor two dirhams for one dirham.'" Yazid said: "Do not [sell] two sa's of dates for one sa', nor two sa's of wheat for one sa'."