Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) "made sadaqat al-fitr obligatory as a purification of the fasts from idle talk and obscenity, and as food for the poor. Whoever pays it before the Eid, it is accepted; and whoever pays it after the prayer, it is an ordinary charity for him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7692