Jabir bin Abdullah (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) forbade "that raisins and dates, or unripe (busr) dates and fresh ripe (rutab) dates, be mixed together to make nabidh."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17455