Musnad Ahmad 30:3114

Waki’ narrated to us, from Sufyan, from Salih, the freed slave of al-Taw’amah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "No people sit in a gathering in which they do not remember Allah, and do not send blessings upon the Prophet, except that it will be a cause of regret for them on the Day of Resurrection."