Musnad Ahmad 30:3147

Narrated to us Abd al-Rahman, from Sufyan, from Salih, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "No people sit in a gathering in which they do not remember their Lord, nor do they invoke blessings upon their Prophet, except that it will be a cause of regret for them on the Day of Resurrection. If He wills, He will punish them for it, and if He wills, He will pardon them."