Musnad Ahmad 30:606

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri—and Ma'mar used to say: from Abu Hurayrah—then he said afterwards: from al-A'raj, from Abu Hurayrah, regarding Zakat al-Fitr: "Upon every free person and slave, male or female, young or old, poor or rich, is a Sa' of dates or half a Sa' of wheat." Ma'mar said: And it reached me that al-Zuhri used to attribute it to the Prophet, may Allah bless him and grant him peace.