It is incumbent on every Muslim to believe in the Qadaa and Qadar, that whatever happens, does so with the Qadaa and Qadar of Allah. By rejecting the Qadaa and the Qadar the person is not considered a Muslim, for believing in it, is an article of faith.
Qadaa means the infinite knowledge of Allah in the Azal (the infinite knowledge of Allah with respect to eternity, concerning every thing). The Qadar is the bringing into existence of these things in accordance to Allah's infinite knowledge, e.g., it is within Allah's knowledge that a certain person will come into existence; this knowledge of Allah concerning this person is called Qadaa, and the process the person is going through until he comes into being is called the Qadar.We must also believe that we are not forced, coerced, or compelled, but that we have a free will and a free choice, to do or not to do, to speak or not to speak and to believe or not to believe. This free will and choice of ours is called Kas'b, and it is for this Kas'b (choice) of ours that we are rewarded