Abdullah bin Ka'b bin Malik (may Allah be pleased with him) narrates from a woman (may Allah be pleased with her) that she heard the Prophet (peace and blessings be upon him) say: "Do not make nabidh by mixing dates and raisins; make each of them separately."
The Shaykh said: The prohibition of mixing the two can carry two meanings. One is that he (peace and blessings be upon him) forbade it merely because the two are mixed, whether or not it reaches the point of intoxication, and permitted drinking it when each is made into nabidh separately. The other is that he (peace and blessings be upon him) forbade it because the mixture becomes strong sooner, while each made separately becomes strong later; so as long as it has not reached the state of strength, in either case, it is not forbidden. And the narration that follows indicates this second meaning.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17462