Musannaf Ibn Abi Shayba 35:138

Hafs ibn Ghiyath narrated to us, from Ibn Jurayj, from ‘Ata’, from Jabir, who said: The Prophet (may Allah bless him and grant him peace) forbade that dates and raisins be steeped together, and that unripe dates (busr) and ripe dates (tamr) be steeped together.