Musnad Ahmad 32:656

Khalf ibn al-Walid narrated to us, Abu Ja'far narrated to us, from Humayd al-Tawil, from Anas ibn Malik, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade that dates and raisins be steeped together, and that dates and unripe dates be steeped together."