Sahl Bin Abdillaah A-Tustaree (rahimahullaah) said:

Abandonment of a command is greater in the sight of Allaah than violating a prohibition; because Aadam was prohibited from eating from the tree and he ate from it, and Allaah forgave him.  Iblees was commanded to prostrate to Aadam and he did not prostrate, and Allaah did not forgive him. [Madaarij As-Saalikeen: 2/156]