Musnad Ahmad 30:3549

Narrated to us Rawh, narrated to us Hammad, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "No people sit in a gathering, then disperse without the remembrance of Allah, except that they disperse like the carcass of a donkey, and that gathering will be a source of regret for them on the Day of Resurrection."