Musnad Ahmad 27:890

Sulayman ibn Dawud al-Hashimi narrated to us, Sa'id ibn 'Abd al-Rahman al-Jumahi narrated to us, from 'Ubayd Allah ibn 'Umar, from Nafi', from Ibn 'Umar: "The Messenger of Allah (peace and blessings of Allah be upon him) obligated Zakat al-Fitr from Ramadan as a sa' of dates or a sa' of barley, upon every free person or slave, male or female, from among the Muslims."