Ja'far bin Burqan says: The letter of Umar bin Abd al-Aziz (may Allah have mercy on him) came to us, saying: Pay Sadaqat al-Fitr before the Eid prayer. "He has certainly succeeded who purifies himself, and mentions the name of his Lord and prays" [Surah al-A'la: 14-15]. And say as your father said: "Our Lord, we have wronged ourselves, and if You do not forgive us and have mercy upon us, we will surely be among the losers" [Surah al-A'raf: 23]. And say as Nuh (peace be upon him) said: "And unless You forgive me and have mercy upon me, I will be among the losers" [Surah Hud: 47]. And say as Ibrahim (peace be upon him) said: "And it is He who, I hope, will forgive me my sin on the Day of Recompense" [Surah al-Shu'ara: 82]. And say as Musa (peace be upon him) said: "My Lord, indeed I have wronged myself, so forgive me; and He forgave him. Indeed, He is the Forgiving, the Merciful" [Surah al-Qasas: 16]. And say as Yunus (peace be upon him) said: "There is no deity except You; exalted are You. Indeed, I have been of the wrongdoers" [Surah al-Anbiya: 87]. I think he also wrote that whoever has nothing to give as Sadaqat al-Fitr should fast after Eid. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7740