Don't laugh too much
لَا تُكْثِرُوا الضَّحِكَ، فَإِنَّ كَثْرَةَ الضَّحِكِ تُمِيتُ الْقَلْبَ
La tukthiru-d-dahik, fa-inna kathrata-d-dahiki tumitu-l-qalb.
"Do not laugh excessively, too much laughter deadens the heart."
Narrated by Abu Hurayrah. Jami' at-Tirmidhi 2305 (hasan)
A short reflection
Not a prohibition on joy. A warning about a posture, the one that never takes anything seriously. Something inside stops feeling.