Maliks Muwatta Book 17, Hadith Number 54.

Yahya related to me from Malik from Zayd ibn Aslam from lyad ibn Abdullah ibn Sad ibn Abi Sarh al-Amiri that he had heard Abu Said al-Khudri say, “We used to pay the zakat al-fitr with a sa of wheat, or a sa of barley, or a sa of dates, or a sa of dried sour milk, or a sa of raisins, using the sa of the Prophet, may Allah bless him and grant him peace.”