Kehf 50:

•  Hatırlayın ki , meleklere :' Adem'e secde edin ' diye emretmiştik te, hepsi hemen secdeye kapanmış, fakat iblis secde etmemişti. O cinlerdendi ve Rabbisinin emri dışına çıkmıştı. Böyle iken siz, Allah(cc)'ı bırakıp ta, size düşman oldukları halde onu ve soyunu dost, işlerinizi havale edeceğiniz vekil ve başınızda veli mi edineceksiniz? Zalimler için ne kötü bir takas!.
  (
  It was possible for Iblis to disobey Allah because he was not one of the angels but one of the jinns. It must be noted that the Qur'an is very explicit that the angels are inherently obedient:
  
  (1) "They do not show arrogance; they fear their Lord Who is above them, and do whatever they are bidden." (XVI: 50)
  
  (2) "..... They do not disobey the Command that is given to them by their Lord and do whatever they are bidden to do." (LXVI: 6)
  
  In contrast to the angels, the jinns have been, like human beings, given the option to obey or not to obey. Therefore they have been given the power to believe or disbelieve, to obey or not to obey. This thing has been made explicit here by saying that Iblis was one of the jinns, so he deliberately chose the way of disobedience. Incidentally, this verse removes all those misunderstandings that are generally found among the common people that Iblis was one of the angels and not an ordinary angel but the instructor of the angels. As regards the difficulty that arises because of this statement of the Qur'an, "When We said to the angels, `Bow down before Adam. They bowed down but Iblis did not'," it should be noted that the Command to the angels was meant for all those on the Earth who were under the administration of the angels so that they should also be made submissive to man. Accordingly all these creatures bowed along with the angels but Iblis refused to bow down along with them. ET
  )