Musnad Ahmad 31:474

'Abd al-Malik ibn 'Amr narrated to us, Hisham narrated to us, from Qatadah, from Abu Nadrah, from Abu Sa'id al-Khudri, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade mixing unripe dates with dried dates, and raisins with dried dates."