Allaah, ta`aalaa, said in the Qur’aan:
(إِنَّا كُلَّ شَيْءٍ خَلَقْنَاهُ بِقَدَرٍ)
Innaa kulla shay’in khalaqnaahu biqadar
Aayah 49 of Sooratul-Qamar means: {Allaah creates everything according to His Destining.}
The Prophet, sallallaahu `alayhi wa sallam, said:
الإيمانُ أن تُؤمِنَ باللهِ وَ ملائِكَتِهِ و كُتُبِهِ وَ رُسلِهِ واليَومِ الآخِرِ و تُؤمِنَ بِالقَدَرِ خَيرِهِ و شَرِهِ
This means: “Belief is to believe in Allaah, His angels, His Books, His messengers, the Day of Judgment, and to believe in the qadar (destiny), both the good and the evil.” Related by Muslim.
Believing in the qadar (good and evil) means: to believe that everything that comes into existence, be it good or evil, is by the Eternal Will of Allaah. The good actions of the slave are according to the Will and Acceptance of Allaah.
The evil actions of the slave are according to the Will of Allaah, but not His Acceptance.
Benefit: The Prophet, may Allaah raise his rank, said to Ibn ^Abbaas, may Allaah raise the ranks of both of them:
قالَ رَسُولُ اللهِ صلى الله عليه وسلم لابنِ عَبَّاسٍ رَضيَ الله عَنهُما : واعلَم أنَ الأمَّةَ لو اجتَمَعَت على أن يَنفَعُوك بِشيئٍ لم ينفعُك إلا بشئٍ قَد كَتَبَهُ الله لَكَ ، وَلَو اجتَمَعُوا على أن يَضُرُوك بِشئٍ لَم يَضُرُّكَ إلا بِشَئٍ قَد كَتَبَهُ الله عَلَيكَ رُفِعَتِ الأقلامُ و جَفَّتِ الصُّحُفُ
This means: “Know that if the entire nation agree to benefit you with something, they will not benefit you except with what Allaah has willed for you. Likewise, if they agree to harm you with something, they will not harm you except with what Allaah has willed for you. The Pen has been lifted, and the Tablet has dried.” Related by at-Tirmidhiyy.