Musnad Ahmad 28:614

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Shaddad Abu Talhah al-Rasibi narrated to us, I heard Abu al-Wazi’ Jabir ibn ‘Amr narrating from ‘Abdullah ibn ‘Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no group of people who sit in a gathering in which they do not remember Allah, except that they will see it as a cause of regret on the Day of Resurrection."