I heard the Messenger of Allah ﷺ say this: Ablution is obligatory (for one who takes anything) touched by fire.  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ الْوُضُوءُ مِمَّا مَسَّتِ النَّارُ  

I am performing ablution because of having eaten pieces of cheese, for I heard the Messenger of Allah ﷺ say: Perform ablution (after eating anything) touched by fire.  

أَخْبَرَهُ أَنَّهُ وَجَدَ أَبَا هُرَيْرَةَ يَتَوَضَّأُ عَلَى الْمَسْجِدِ فَقَالَ إِنَّمَا أَتَوَضَّأُ مِنْ أَثْوَارِ أَقِطٍ أَكَلْتُهَا لأَنِّي سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ تَوَضَّئُوا مِمَّا مَسَّتِ النَّارُ  

The Messenger of Allah ﷺ said. Perform ablution (after eating) anything touched by fire.  

سَمِعْتُ عَائِشَةَ زَوْجَ النَّبِيِّ ﷺ تَقُولُ قَالَ رَسُولُ اللَّهِ ﷺ تَوَضَّئُوا مِمَّا مَسَّتِ النَّارُ