Sahih Muslim Book 35, Hadith Number 6520.

Chapter : The merit of an assembly for the recitation of the Qur’an or for the Dhikr (Remembrance of Allah).

Agharr Abi Muslim reported: I bear witness to the fact that both Abu Huraira and Abu Sa’id Khudri were present when Allah’s Messenger may peace be upon him) said: The people do not sit but they are surrounded by angels and covered by Mercy, and there descends upon them tranquillity as they remember Allah, and Allah makes a mention of them to those who are near Him. This hadith has been narrated on the authority of Shu’ba with the same chain of transmitters.