Musnad Abu Ya'la al-Mawsili 42:286

Zuhayr narrated to us, Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, from al-A'mash, from Malik ibn al-Harith, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade mixing unripe dates with ripe dates, and he forbade mixing raisins with dates. I said: "Does this mean they should be steeped together?" He said: "Yes."