There remains a brief statement regarding al-Qadar, and it pertains to the fact that belief in Pre-decree produces magnificent fruits affecting the conduct and the heart of man.
Because if you believe that everything occurs by the Pre-decree of Allaah, then at times of delight you will give thanks to Allaah, The Mighty and Majestic, and you do not become amazed with your own self. You do not also think that this matter occurred by your own power and strength. You do, however, believe that it is just a cause– in case you have done the necessary mean by which you have attained that which delights you–and that all the good is in the Hand of Allaah. So you will be more thankful for the blessing granted by Allaah, The One free of all imperfection, The Most High. This will motivate you to carry on the obedience to Allaah in accordance with that which He commanded you, and that you do not regard your own self having a favor upon Allaah. Rather, you see that the favor is His, The One free of all imperfection, The Most High. He, The Exalted, says:
They regard as favor upon you (O Muhammad Sallallaahu alaihi wa sallam) that they have embraced Islaam. Say: “Count not your Islaam as a favor upon me. Nay, but Allaah has conferred a favor upon you, that He has Guided you to the faith, if you indeed are true.” [Qur’aan, soorat al-Hujuraat (49): 17].
Similarly, if a calamity befalls you, you believe in Allaah and submit without being regretful or afflicted with grief on that. Have you not given thought to the saying of the Prophet (Sallallaahu alaihi wa sallam):
“A strong believer is better and is more lovable to Allaah than a weak believer, and there is good in every one of them. Cherish that which gives you benefit and seek help from Allaah and do not lose heart, and if anything (in the form of trouble) comes to you, do not say: If I had not done that, it would have not happened so and so, but say: Allaah did that which He had Pre-decreed, and He does whatever He Wills, and that “if” opens the way for the acting of Shaytan [by casting opposition to Allaah’s Pre-decree in the heart].” 
So, in the belief in the Pre-decree there is rest to the soul and heart and absence of grief at that which escaped, and of worry and distress at that which is to come. Allaah, The Most High, Says:
No calamity befalls on the earth or in yourselves but it is inscribed in the Book of Decrees (al-Lawh al-Mahfooth) before we bring it into existence. Verily, that is easy for Allaah. In order that you may not grieve at the things that you fail to get, not to rejoice over things which has been given to you.” [Qur’aan, soorat al-Hadeed (57): 22-23].
As to the one who does not believe in al-Qadar, there is no doubt that he will grieve and regret at times of calamities, and the Shaytaan will open up for him every new way or possibility [to confuse him and keep him dissatisfied]. And That he will be pleased and become proud and deluded if prosperity befalls him. The Eemaan in the Pre-decree, however, prevents all of this.
 Reported by Muslim in his Saheeh, vol.4, no.644. The statement between the brackets,“[…]”, is the explanation of al-Qaadee ‘Iyaad in Sharh Saheeh Muslim by An-Nawawee.
Translated by Dr Saleh as Saleh rahimahullaah
Source : Understanding Worship – Fiqh ul-‘Ibadah – Shaykh ibn Uthaymeen – Page 93