Musnad Ahmad 31:195

Yahya narrated to us, from Sa’d ibn Ishaq, who said: Zaynab bint Ka’b ibn ‘Ujrah narrated to me, from Abu Sa’id al-Khudri, who said: A man said to the Messenger of Allah (may Allah bless him and grant him peace): "What do you think of these illnesses that afflict us; what is there for us in them?" He said: "Expiations." My father said: "Even if it is little?" He said: "Even if it is a thorn or something greater than it." He said: So my father prayed against himself that the fever would not leave him until he died, provided that it would not distract him from Hajj, nor ‘Umrah, nor Jihad in the way of Allah, nor the obligatory prayer in congregation. No person touched him except that he felt its heat, until he died.