Musnad Ahmad 30:3074

Waki’ narrated to us, he said: ‘Abd al-‘Aziz—meaning Ibn ‘Abd Allah ibn Abi Salamah—narrated to us, from Ibn Shihab, from ‘Umar ibn ‘Abd al-‘Aziz, from Ibrahim ibn Qariz, who said: I saw Abu Hurairah performing ablution (wudu) above the mosque. I said: "From what are you performing ablution?" He said: "From pieces of dried yogurt (aqt) that I ate. I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Perform ablution from what the fire has touched.'"