Musnad Ahmad 25:1251

Asbat narrated to us, al-Shaybani narrated to us, from Habib ibn Abi Thabit, from Sa'id ibn Jubayr, from Ibn Abbas, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, forbade that unripe dates and dried dates be mixed together, and that raisins and dried dates be mixed together." He said: "And he wrote to the people of Jurash: that they should not mix raisins and dried dates."