Hadith: No people leave a gathering in which they did not remember Allah, the Almighty, except that it will be as if they are leaving the carcass of a donkey, and it will be a cause of regret for them

عن أبي هريرة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «ما من قوم يقومون من مجلس لا يذكرون الله تعالى فيه، إلا قاموا عن مثل جيفة حمار، وكان لهم حسرة».
[صحيح] - [رواه أبو داود]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "No people leave a gathering in which they did not remember Allah, the Almighty, except that it will be as if they are leaving the carcass of a donkey, and it will be a cause of regret for them."
Sahih/Authentic. - [Abu Dawood]

Explanation

Whoever sits in a gathering where Allah, the Almighty, is not mentioned is like one who sits at a dining table where the carcass of a donkey is served, which is the most disgusting and putrid food imaginable. This is a strong metaphor to indicate the hideousness of neglecting the remembrance of Allah. It is a cause of severe regret for the time wasted in idle talk that entails no benefit at all. Hence, Muslims should be extremely keen on filling their gatherings with acts of worship and obedience to Allah, and should run away from idle gatherings of amusement just as they run away from filth and dirt. Indeed, every person is responsible for his time and will be held accountable for the way he spent it; if it was spent in a good way, then he would get rewarded for it, otherwise, he would bear the evil consequences.