Musnad Abu Ya'la al-Mawsili 138:7

Zuhayr narrated to us, Waki' narrated to us, Hasan ibn Salih narrated to us, from Khalid ibn al-Fazar, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Beware, indeed the *muzatt* (mixtures) are forbidden: the mixing of unripe dates and dried dates."