Musnad Abu Ya'la al-Mawsili 154:80

Ibn Numayr narrated to us, my father narrated to us, Fudayl ibn Ghazwan narrated to us, Abu Dahqanah narrated to us, he said: I was sitting with Ibn Umar. He said: A guest came to the Messenger of Allah, may Allah bless him and grant him peace, so he said to Bilal, "Bring us some food." Bilal went and exchanged two sa's of dates for one sa' of better dates, and their dates were of poor quality. The Prophet was pleased with it, so he said, "Where did this come from?" He informed him that he had exchanged two sa's for one sa'. The Prophet, may Allah bless him and grant him peace, said, "Return our dates to us."