Musannaf Ibn Abi Shayba 1:562

Waki’ narrated to us, from ‘Umar ibn Shabbah, from ‘Abdullah ibn Shabbah, from ‘Abdullah ibn Ibrahim, who said: I was with Abu Hurayrah, and he performed ablution above the mosque. I said to him, "For what thing did you perform ablution?" He said, "I ate a piece of dried yogurt (aqit)."