Musnad Ahmad 31:79

Yahya ibn Sa'id narrated to us, he said: Sulayman al-Taymi narrated to us, Abu Nadra narrated to us, he said: Abu Sa'id al-Khudri narrated to me, from the Prophet, peace and blessings of Allah be upon him: "That he forbade the earthen jar to be used for making nabidh (fermented drink) in it, and that dates and raisins be mixed together, and that unripe dates (busr) and ripe dates (tamr) be mixed together."