It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) forbade mixing dates and raisins, or unripe and ripe dates together (to make nabidh).
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12599
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد حسن، وانظر: 12423