Musnad Ahmad 31:931

Waki' narrated to us, Dawud ibn Qays al-Farra' narrated to us, from 'Iyad ibn 'Abd Allah ibn Abi Sarh, from Abu Sa'id al-Khudri, who said: "We used to bring out the Sadaqat al-Fitr when the Messenger of Allah (peace and blessings of Allah be upon him) was among us as a sa' of food, or a sa' of dates, or a sa' of barley, or a sa' of raisins, or a sa' of dried yogurt (aqit). We continued in that manner until Mu'awiyah came to us."