Musnad Ahmad 30:2392

Isma'il narrated to us, he said: Ma'mar narrated to us, from al-Zuhri, from Umar ibn Abd al-Aziz, from Ibrahim ibn Abdullah ibn Qariz — or Qarid, I do not know, Isma'il doubted — that Abu Hurayrah ate pieces of dried yogurt (aqt), then he performed ablution. He said: "Do you know why I performed ablution? I ate pieces of dried yogurt, so I performed ablution, for I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Perform ablution from what the fire has touched.'"