Indeed when a person faces calamities with patience [but] without hoping for a reward from Allaah, the calamity will become an expiation for his sins. 
And if he shows patience along with hoping for a reward [from Allaah], then along with being an expiation for his sins it becomes a "good recompense and reward". 
And the meaning of hoping for Allaah’s reward [al-Ihtisaab] is that a person believes that he will soon be rewarded for the patience [he is showing], so he makes good his thoughts about Allaah and thus Allaah, the Mighty and Majestic, will give him [a reward] according to what he thought of Him.

 
Shaikh Ibn Uthaimeen (rahimahullah)