Musnad Ahmad 30:2636

Waki’ narrated to us, from Sufyan, from Salih—meaning the freed slave of al-Taw’amah—from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "No people gather in a sitting and then disperse without remembering Allah (the Mighty and Majestic) and sending blessings upon the Prophet (peace and blessings of Allah be upon him), except that their sitting will be a cause of regret for them on the Day of Resurrection."