Abu Hurairah Radhi allahu anhu narrated that :

the Prophet Sal-Allahu Alaihi Wasallam said: “No group gather in a sitting in which they do not remember Allah, nor sent Salat upon their Prophet, except it will be a source of remorse (guilt) for them. If He wills, He will punish them, and if He wills, He will forgive them.”

Jamia Tirmidhi Vol. 6, Book 45, 3380-Sahih