Musannaf Ibn Abi Shayba 35:139

Ibn Mushir narrated to us, from al-Shaybani, from Habib, from Sa'id ibn Jubayr, from Ibn Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade mixing dates and raisins together, and he forbade mixing unripe dates and raisins together. He wrote regarding that to the people of Jurash.