Musannaf Ibn Abi Shayba 35:141

Ibn Numayr narrated to us, from al-A’mash, from Habib ibn Abi Arta’ah, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade mixing unripe dates with ripe dates, and raisins with dates. And he mentioned that Abu Hanifa said: There is no harm in it.