Anas b. Mālik, may Allāh be pleased with him, reported that the Prophet ﷺ said, “If there were 100,000 or more in this masjid, and among them was a man from the people of the Fire, and he were to breathe and his breath were to touch them: the masjid and those in it would be burnt.” Reported by Abū Yaʿlá.

Graded ṣaḥīḥ by al-Albānī in Ṣaḥīḥ al-Targhīb (3668).

And al-Bazzār narrated it with the wording:

“If there were 100,000 or more in the masjid, then a man from the people of the Fire were to breathe, he would have burnt them all.”

Graded ṣaḥīḥ li ghayrih by al-Albānī.