Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) said that he asked: "O Messenger of Allah! What is the reward for fever?" He (peace and blessings be upon him) said: "As long as a person's foot stumbles or a vein throbs with pain, (the reward of) his good deeds continues to flow for him." Sayyiduna Ubayy (may Allah be pleased with him) (supplicated): "O Allah, I ask You for a fever that does not prevent me from going out in Your path, from going to Your House (for Hajj), or to the Mosque of Your Prophet." The narrator said: Thereafter not a single evening passed for Sayyiduna Ubayy (may Allah be pleased with him) without his having a fever.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 537