Musnad Ahmad 30:2835

Narrated to us Abd al-Rahman, from Shu'bah, from al-A'mash, from Abu Salih, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "No people sit in a seat in which they do not remember Allah, the Mighty and Majestic, and do not send blessings upon the Prophet, peace and blessings of Allah be upon him, except that it will be a cause of regret for them on the Day of Resurrection, even if they enter Paradise for the reward."