Abu Qatadah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) forbade making nabidh by mixing dates with unripe dates, and dates with raisins, and ordered: "Make the nabidh of each one separately."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17456