Sahih Muslim Book 35, Hadith Number 6521.
Abu Sa’id Khudri reported that Mu’awiya went to a circle in the mosque and said: What makes you sit here? They said: We are sitting here in order to remember Allah. He said: I adjure you by Allah (to tell me whether you are sitting here for this very purpose)? They said: By Allah, we are sitting here for this very purpose. Thereupon, he said: I have not demanded you to take an oath, because of any allegation against you and none of my rank in the eye of Allah’s Messenger (may peace be upon him) is the narrator of so few ahadith as I am. The fact is that Allah’s Messenger (may peace be upon him) went out to the circle of his Companions and said: What makes you sit? They said: We are sitting here in order to remember Allah and to praise Him for He guided us to the path of Islam and He conferred favours upon us. Thereupon he adjured by Allah and asked if that only was the purpose of their sitting there. They said: By Allah, we are not sitting here but for this very purpose, whereupon he (the Messenger) said: I am not asking you to take an oath because of any allegation against you but for the fact that Gabriel came to me and he informed me that Allah, the Exalted and Glorious, was talking to the angels about your magnificence.