Musnad Ahmad 30:2622

Waki’ narrated to us, he said: ‘Ikrimah ibn ‘Ammar narrated to us, from Abu Kathir al-Hanafi, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) forbade mixing raisins and dates, and unripe dates and ripe dates. And he said: "Each one of them should be steeped separately."