Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) made sadaqat al-fitr of Ramadan obligatory upon the people: "A sa' of dates or a sa' of barley, on behalf of every slave and free person, male and female, among the Muslims."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7687