He found Abu Hurayrah performing ablution at the mosque, so Abu Hurayrah said, "I am only performing ablution because of pieces of dried curd I ate, for I heard the Messenger of Allah ﷺ say, 'Perform ablution after eating anything touched by fire.'"
أنه وجد أبا هريرة يتوضأ على المسجد، فقال: إنما أتوضأ من أثوار أقط أكلتها؛ لأني سمعت رسول الله ﷺ يقول: توضئوا مما مست النار.
Muslim narrated it in the chapter on Menstruation (352) from the hadith of Ibn Shihab, from Umar bin Abdul-Aziz, that Abdullah bin Ibrahim bin Qaridh informed him... then he mentioned the hadith.Aqit is hardened, solidified milk. Athwar is the plural of thawr, which means a piece of aqit.Abu Hurairah was firm in narrating this hadith. In the Sunan collections it reads, "Wudu is required from whatever fire has touched, even if it is only a piece of aqit." Ibn Abbas said to him, "O Abu Hurairah, should we perform wudu after oil? Should we perform wudu after hot water?" Abu Hurairah replied, "O son of my brother, when you hear a hadith from the Messenger of Allah ﷺ, do not draw comparisons for it."Tirmidhi narrated it this way (79), and Ibn Majah narrated it in abridged form (485), both through the route of Sufyan bin Uyayna, from Muhammad bin Amr bin Alqama, from Abu Salama, from Abu Hurairah. Its isnad is sahih.In Sunan Nasai (174), when Ibn Abbas said, "Should I perform wudu because of food that I find lawful in the Book of Allah, simply because fire has touched it?" Abu Hurairah gathered a handful of pebbles and said, "I testify, by the number of these pebbles, that the Messenger of Allah ﷺ said, 'Perform wudu from whatever fire has touched.'"Abu Dawud narrated it (194) through another route, from Aghar, from Abu Hurairah, with the wording, "Wudu is required from whatever fire has cooked."As for Ibn Abbas making these comparisons, that was because he had witnessed the Prophet ﷺ eat a sheep's shoulder without performing wudu afterward, which would make this an abrogation of the Prophet's ﷺ statement, "Perform wudu from whatever fire has touched." And what Ibn Abbas held is correct.Perhaps Abu Hurairah did not remember this at first, then later remembered, and narrated the hadith stating that he had seen the Prophet ﷺ perform wudu after eating pieces of aqit, and then later saw him eat a sheep's shoulder and pray without performing wudu. Its sourcing will come in the chapter that follows, Allah willing.