Sahih Muslim Book 23, Hadith Number 4907.

Chapter : It is not approved to prepare Nabidh by mixing dry dates and grapes.

‘Abdullah b. Abu Qatada, on the authority of his father, reported Allah’s Apostle (may peace be upon him) 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.