From Sahih Muslim (Hadith 5158):
Abdullah b. Abu Qatada, on the authority of his father, reported Allah's Apostle (ﷺ) forbidding the preparation of the mixture of ripe dates and unripe dates, and the mixture of grapes and dates, and that of nearly ripe dates and fresh dates but the Prophet said:Prepare the Nabidh from each one of them separately
Reference: Muslim 5158