Musannaf Ibn Abi Shayba 3:3207

'Abd al-Rahim ibn Sulayman narrated to us, from Hajjaj, from 'Ata', from Ibn 'Abbas, who said: "Indeed, it is from the Sunnah that the Sadaqat al-Fitr be brought out before the prayer, and it is not to be brought out until you have fed [the poor]."