Musnad Ahmad 31:607

Abu Sa’id and Mu’awiyah narrated to us, they said: Za’idah narrated to us, Al-A’mash narrated to us, from Malik ibn al-Harith, from Abu Sa’id al-Khudri, who said: "The Messenger of Allah (may Allah bless him and grant him peace) forbade mixing dates and raisins, and he forbade mixing unripe dates (zahw) and ripe dates." I said to Sulayman: "That they be steeped together?" He said: "Yes."