Musnad Ahmad 1007:130

Yahya ibn Sa'id narrated to us, from Hisham, Yahya ibn Abi Kathir narrated to us, from 'Abdullah ibn Abi Qatadah, that he heard his father [say] that the Prophet, peace and blessings of Allah be upon him, forbade making nabidh (a fermented drink) from fresh dates and unripe dates together, and from dried dates and raisins together. And he said: "Make nabidh from each one of them separately."