I have a daughter who, since birth, has suffered from a disease which is not serious but cannot be cured. I always supplicate Allaah The Almighty to cure her. However, sometimes when her condition worsens, I lose patience and say, “O My Lord, why all this suffering?” and I say other things that I hope Allaah The Almighty forgives me for. Is this considered an act of disbelief? How can I repent?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

A Muslim must submit to the Decree and Fate of Allaah The Almighty in everything. He must beware not to object to the Decree and Fate of Allaah The Almighty or show despair because of it. Indeed, all evil lies in such behavior while all good lies in the choice and Decree of Allaah The Almighty. If Allaah The Almighty decrees for the slave something that he is displeased with such as disease, calamities, afflictions in the self, family and children, then He wills what is good for His slave by them, but man is indeed unjust  and ignorant.

On the authority of Suhayb ibn Sinaan the Prophet, sallallaahu ‘alayhi wa sallam, said that all the states and conditions of the believer are good for him. If he is in a state of well-being, he thanks Allaah The Almighty and that is good for him. If an affliction befalls him, he is patient and that is also good for him. [Muslim]

Afflictions are a test from Allaah The Almighty to His slaves in order to distinguish the true believers from the false ones. Therefore, entrust your affairs and the affairs of your daughter to Allaah The Almighty and do not say anything but good words. Objecting to Fate may lead you to disbelief. You should seek the forgiveness of Allaah The Almighty for what you said, submit to His Decree and ask Him to remove the affliction and ward off harm from your daughter; for He is the Most Generous, the Most Bountiful.

And Allaah Knows best.