Yahya related to me from Malik from Nafi from Abdullah ibn Umarthat the Messenger of Allah, may Allah bless him and grant him peace,made the zakat of breaking the fast at the end of Ramadan obligatoryon every muslim, whether freeman or slave, male or female, andstipulated it as a sa' of dates or a sa' of barley.
USC-MSA web (English) reference: Book 17, Hadith 53
Arabic reference: Book 17, Hadith 629