Musnad Ahmad 27:2007

Muhammad ibn Isma'il ibn Abi Fudayk narrated to us, al-Dahhak—meaning ibn 'Uthman—narrated to us, from Nafi', from Ibn 'Umar: "From the Prophet, may Allah bless him and grant him peace, that he commanded the giving out of Zakat—Zakat al-Fitr—that it be performed before the people go out to the prayer."