Musnad Ahmad 27:1969

Yahya ibn Adam and Humayd ibn Abd al-Rahman al-Ru'asi narrated to us, they said: Zuhayr narrated to us, Musa ibn Uqbah narrated to us, Nafi' informed me, from Abdullah ibn Umar, that he used to narrate that the Messenger of Allah, peace and blessings of Allah be upon him: "Ordered that Zakat al-Fitr be paid before the people go out to the prayer."