It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that a man said: O Messenger of Allah! What is your view about these illnesses that afflict us; what will we get in return for them? He (peace and blessings be upon him) said: "They are an expiation for sins." Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) said: O Messenger of Allah! Even if they are slight? He (peace and blessings be upon him) said: "Even the prick of a thorn or anything greater." (Hearing this,) Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) prayed for himself that fever would not leave him until death, on the condition that it would not keep him from Hajj, Umrah, jihad in the path of Allah, and the obligatory prayer in congregation. So after that, (because of the intensity of the fever) whenever anyone touched his body, he would feel its heat, until he passed away. This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.