Happiness and Misery

The Prophet of Allaah (sallAllaahu 'alayhi wa sallam) said:

"The happiness of the son of Adam depends on his being content with what Allaah has decreed for him, and the misery of the son of Adam results from his failure to pray istikhara, and the misery of the son of Adam results from his discontentment with what Allaah has decreed for him."

[Tirmidhee, classified saheeh by al-Haakim and al-Dhahabi]