Musnad Ahmad 27:1929

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, Musa ibn 'Uqbah informed me, from Nafi', from Ibn 'Umar: that he narrated "that the Messenger of Allah, may Allah bless him and grant him peace, commanded that Zakat al-Fitr be paid before the people go out to the prayer ground." And he said: "Once, [he said] 'before the prayer'."