Anas bin Malik (May Allah be pleased with him) reported: Messenger of Allah (Sallallahu Alayhi Wa Sallam) delivered a Khutbah to us the like of which I had never heard from him before. He said, "If you knew what I know, you would laugh little and weep much". Thereupon those present covered their faces and began sobbing. [Al-Bukhari and Muslim]

Another narration is: Messenger of Allah (Sallallahu Alayhi Wa Sallam) heard of something about his Companions upon which he addressed them and said, "Jannah and (Hell) Fire were shown to me and I have never seen the like of this day in good and in evil. If you were to know what I know, you would laugh little and weep much". His Companions experienced such suffering on that day that had no equal. They covered their faces and began to weep. [Al-Bukhari]

Commentary:
1. We learn from this Hadith that Jannah and Hell do exist. The Prophet (Sallallahu Alayhi Wa Sallam) was shown both of them.
2. Excessive laughing is improper because it indicates that one is careless and forgetful of the Hereafter, while a Muslim is required to be all the time alert and careful about it.
3. To weep out of fear of Allah, is praiseworthy because it indicates that one's heart is full of fear and he is worried about the Hereafter.