It is an obligation to believe in the Destining (Qadar), that is, to believe that everything that happens is according to the Will of Allah, whether the destined matter is good or evil. The good deeds of the slaves are by Allah’s destining, His acceptance, and blessing. The evil deeds of the slaves are by Allah’s destining without His reward or blessings.
The Prophet, salla-l-lahu ^alayhi wa sallam, said:
الإِيمانُ أنْ تُؤْمِنَ بِاللهِ وَمَلائِكَتِهِ وَكُتُبِهِ وَرُسُلِهِ وَاليَومِ الآخِرِ وَتُؤمِنَ بِالقَدَرِ خَيرِهِ وَشَرّهِ
This means: “Iman is to believe in Allah, and His Angels, and His Revealed Books, and His Messengers, and the Last Day; and to believe in the Qadar (destiny), its good and its evil (that is, both are decreed by Allah).” Narrated by Muslim.