Musnad Ahmad 30:1931

'Affan narrated to us, Wuhayb narrated to us, Suhayl narrated to us, from his father, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "No people gather and then disperse without the remembrance of Allah, except that it is as if they dispersed from the carcass of a donkey, and that gathering will be a source of regret for them."