Sayyiduna Ibn Umar (may Allah be pleased with them both) used to say: "It has been forbidden to make nabidh from unripe dates (busr) and fresh dates (rutab) together, and it has also been forbidden to make nabidh from dried dates and raisins together."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13351