Musnad Abu Ya'la al-Mawsili 160:210

Muhammad ibn Bakkar narrated to us, Abu Ma'shar narrated to us, from Sa'id al-Maqburi, who said: We saw Abu Hurayrah performing ablution on the roof of the mosque. I said: "O Abu Hurayrah, from what are you performing ablution?" He said: "I ate a piece of dried yogurt (aqit). I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'Perform ablution from what the fire has changed.'"