Musnad Ahmad 1117:1

Muhammad ibn Salamah narrated to us, from Muhammad ibn Ishaq, from Ma'bad ibn Ka'b ibn Malik, from his mother—and she had prayed toward the two qiblahs with the Messenger of Allah (peace and blessings of Allah be upon him)—she said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) forbidding that dates and raisins be steeped together. And he said: "Steep each one of them separately."