Musnad Ahmad 32:633

Aswad narrated to us, al-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, the *muzāt* are forbidden." And the *muzāt* is the mixing of dates and unripe dates.