Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Whenever a people sit in a gathering in which they do not remember their Lord and do not send blessings upon their Prophet (peace and blessings be upon him), it will be a cause of regret for them on the Day of Resurrection; if Allah wills He will take them to account, and if He wills He will forgive them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5772