Do not revile the fever, for it cleanses out the sins of the children of Adam in the same way that a furnace removes the dirt of iron
Jabir (May Allah be pleased with him) reported:
The Messenger of Allah (may the peace and blessings of Allah be upon him) visited Umm As-Sa'ib (or Umm Al-Musaiyyab) and asked her, "What ails you O Umm As-Sa'ib (or Umm Al-Musaiyyab)? You are shivering." She replied: "It is a fever, may Allah not bless it!" He said to her, "Do not revile the fever, for it cleanses out the sins of the children of Adam in the same way that a furnace removes the dirt of iron." [Muslim].
The Prophet (may the peace and blessings of Allah be upon him) used to visit his companions due to his tenderness and caring for them. He used to accept their invitations.
He had a special care on visiting the sick so as to comfort them and console their hearts, to bring pleasure to them, and pray for them.
The Fever and the like diseases removes the sins of the children of Adam little by little as long as it exists, until one is completely purified on the condition that one is patient and expects the reward from Allah.
The similitude in this Hadith is very precise and eloquent. The furnace completely remove the rust of the iron, until it looks like new, and so the fever acts like the fire which continues to clean the sins as long as Allah wills. It is like a gift that followed the test..
This Hadith teaches us to be polite with Allah the Almighty, we may not utter a word that indicates dissatisfaction with fate, even indirectly. One may not ever utter any word that entails despair. It would have a negative effect on those around him who may reject his words or hate him. It would be worse if the listener is a flunkey who imitates this falsehood.