Essentially part of Tawheed (Unity of Allah). Belief that God is Just. He will reward or punish any person according to his deeds and thus the notion of predestination (where all decisions are God's and not any one else's) in one's deeds does not exist.
It is absolutely forbidden in Islam to believe that the Almighty, Merciful Allah planned our destiny and that the good and the bad are just His Will and there is no choice for us between them (God forbid!).
Those who ascribe to such lies do so because they want to blame their own evils on Allah and claim the good for themselves!
Although the attribute of Allah's Justice is not a separate entity of Allah
for certainly the Almighty God can never be compartmentalized nor defined in
any relative terms, it is nonetheless absolutely compulsory to believe and fully
understand the importance of this attribute in Islam so as not to allow the
evil suggestions of Iblees (curse of Allah be upon him and his progeny) and
his companions from leading us astray.