Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) "ordered the payment of Sadaqat al-Fitr, that it be paid before the people go out to the Eid prayer ground," and Abdullah (may Allah be pleased with him) used to pay it one or two days before that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7738