Musnad Abu Ya'la al-Mawsili 42:166

Zakariya ibn Yahya al-Kisa'i narrated to us, Abu Bakr ibn 'Ayyash narrated to us, from al-A'mash, from Malik ibn al-Harith, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not mix the *zahw* (fresh, unripe dates) and the dates."