Musnad Ahmad Hadith 11183

It is narrated from Abu Saeed Khudri (may Allah be pleased with him) that a man asked the Prophet (ﷺ), "Tell us, what is there for us regarding the illnesses that afflict us?" The Prophet (ﷺ) said, "These illnesses become an expiation for sins." Ubayy bin Ka'b (may Allah be pleased with him) asked, "Even if the illness is minor?" The Prophet (ﷺ) said, "Yes! Even if a thorn pricks or even less than that." Upon hearing this, Ubayy bin Ka'b (may Allah be pleased with him) prayed for himself that fever should never leave him until death, but it should be such that it does not hinder him from Hajj and Umrah, striving in the way of Allah, and obligatory congregational prayers. Thus, after that, whoever touched him would feel his body hot, until he passed away.