Sahih Muslim
The Book of Drinks (Kitab Al-Ashriba)
Book Number: 023 / Hadith Number: 4910
Ibn 'Abbas reported that Allah's Apostle (may peace be upon him) forbade the mixing of dates and grapes together, and mixing of unripe dates and ripe dates together (for preparing Nabidh), and he wrote to the people of Jurash (in Yemen) forbidding them to prepare the mixture of dates and grapes. This hadith has been narrated through another chain of transmitters with a slight variation of words.