Musnad Ahmad 1167:25

'Attab ibn Ziyad narrated to us, he said: 'Abdullah—meaning Ibn al-Mubarak—narrated to us, he said: Ibn Lahi'ah informed us, from Muhammad ibn 'Abd al-Rahman ibn Nawfal, from Fatimah bint al-Mundhir, from Asma' bint Abi Bakr, she said: "We used to pay the Zakat al-Fitr during the time of the Messenger of Allah, may Allah bless him and grant him peace, as two mudds of wheat, by the mudd that you use for your sustenance."