Musnad Ahmad 30:2715

Hajjaj and Yazid narrated to us, they said: Ibn Abi Dhi'b informed us, from Salih, the freed slave of al-Taw'amah, from Abu Hurairah, that the Prophet, peace and blessings of Allah be upon him, said: "No people sit in a gathering in which they do not remember Allah, nor do they send blessings upon their Prophet, except that it will be a cause of regret for them."