Musnad Ahmad 30:557

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, Ibn Shihab narrated to me, 'Umar ibn 'Abd al-'Aziz informed me, that 'Abd Allah ibn Ibrahim ibn Qariz informed him that he found Abu Hurayrah performing ablution on the roof of the mosque. Abu Hurayrah said: "I am only performing ablution because of some pieces of dried yogurt (aqt) that I ate, because the Messenger of Allah, may Allah bless him and grant him peace, said: 'Perform ablution from what the fire has touched.'"