It is narrated from Sayyiduna Anas (may Allah be pleased with him) that he said: "I went to Sayyiduna Abu al-Darda' (may Allah be pleased with him) to visit him in his illness, and I said: O Abu al-Darda', would you not like to become healthy and then not fall ill? He said: I heard the Messenger of Allah (peace and blessings be upon him) say: 'Indeed, headache and the heat of fever cling to a person until not even a sin the weight of a mustard seed remains of his sins.'"
Hadith Referenceالمعجم الأوسط / باب الألف / 634
Hadith Takhrij«أخرجه أحمد فى «مسنده» برقم: 22150، 22142، والطبراني فى «الأوسط» برقم: 3119»