Rules for Joking in Islam

The Prophet (peace and blessings of Allaah be upon him) said: “Do not laugh too much, for laughing too much deadens the heart.” (Saheeh al-Jaami’, 7312)

‘Umar ibn al-Khattaab (may Allaah be pleased with him) said: “Whoever laughs too much or jokes too much loses respect, and whoever persists in doing something will be known for it.”

The Prophet (peace and blessings of Allaah be upon him) said: “If you knew what I know, you would laugh little and weep much.” In Fath al-Baari it says: “What is meant by knowledge here has to do with the might of Allaah and His vengeance upon those who disobey Him, and the terrors that occur at death, in the grave and on the Day of Resurrection).

Obtained from http://www.islamisforyou.com