Musnad Ahmad 31:194

Yahya narrated to us, from Dawud—meaning Ibn Qays—from Iyad, from Abu Sa’id, who said: "We used to give out Zakat al-Fitr during the time of the Messenger of Allah, peace and blessings of Allah be upon him, as a sa’ of dates, or barley, or dried yogurt (aqit), or raisins."