Musnad Ahmad 30:487

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from 'Umar ibn 'Abd al-'Aziz, from Ibrahim ibn 'Abd Allah ibn Qariz, who said: I passed by Abu Hurayrah while he was performing ablution (wudu'). He said: "Do you know what I am performing ablution from? From pieces of dried yogurt (aqt) that I ate. For I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Perform ablution from what fire has touched.'"