Musnad Ahmad 27:281

Ibn Numayr narrated to us, Fudayl—meaning Ibn Ghazwan—narrated to us, Abu Dahqanah narrated to me, he said: I was sitting with Abdullah ibn Umar, and 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 good dates, and their dates were of inferior quality. The Prophet, may Allah bless him and grant him peace, was pleased with the dates, so the Prophet, may Allah bless him and grant him peace, said: "Where did this date come from?" He informed him that he had exchanged one sa' for two sa's. The Messenger of Allah, may Allah bless him and grant him peace, said: "Return our dates to us."