Musannaf Ibn Abi Shayba 5:536

Hushaym narrated to us, from Sufyan ibn Husayn, from al-Zuhri, from Sa’id ibn al-Musayyib, who attributed it to the Prophet (marfu’): That he was asked about Sadaqat al-Fitr, and he said: "For the young and the old, and the free and the slave, half a sa’ of wheat, or a sa’ of dates or barley."