Sayyidah Umm Ma'bad bin Ka'b (may Allah be pleased with her) said: I heard the Messenger of Allah (peace and blessings be upon him) forbid mixing two things (when making nabidh), that is, mixing dry dates with unripe and fresh dates, and he (peace and blessings be upon him) said: "Drink each of them (prepared) separately."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21815