Musnad Abu Ya'la al-Mawsili 138:6

Abu Bakr ibn Abi Shaybah narrated to us, Muhammad ibn Bishr narrated to us, from Hasan ibn Salih, from Khalid ibn al-Fazar, from Anas ibn Malik, 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 (*busr*) and ripe dates (*tamr*)."