It is narrated from Hazrat Abu Umamah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Fever is the effect of the furnace of Hell. If a Muslim gets fever, then that is his portion from Hell (which is given to him in this world and in the Hereafter he is saved from Hell).
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 22274
Hadith Gradingحکم دارالسلام:حسن لغيره، أبو حصين إن كان هو مروان بن رؤية الشامي فهو ثقة، وإن كان هو راو آخر فهو مجهول