Musnad Abu Ya'la al-Mawsili 245:34

Ahmad ibn Ibrahim narrated to us, he said: Marhum narrated to me, he said: Abu Na'amah al-Sa'di narrated to us, from Abu Uthman al-Nahdi, from Abu Sa'id al-Khudri, who said: Mu'awiyah went out to a circle in the mosque and said: "What makes you sit?" They said: "We sat to remember Allah, the Mighty and Majestic." He said: "By Allah, nothing made you sit except that?" They said: "By Allah, nothing made us sit except that." He said: "As for me, the Messenger of Allah (peace and blessings of Allah be upon him) went out to a circle of his companions and said: 'What makes you sit?' They said: 'We sat to remember Allah, the Mighty and Majestic, and to praise Him for guiding us to Islam and bestowing it upon us.' He said: 'By Allah, nothing made you sit except that?' They said: 'By Allah, nothing made us sit except that.' He said: 'As for me, I did not ask you to swear out of suspicion of you, but Gabriel came to me and informed me that Allah is boasting of you to the angels.'"