Musnad Abu Ya'la al-Mawsili 117:183

Ishaq ibn Abi Isra'il narrated to us, Zuhayr ibn Ishaq narrated to us, Abu Khalaf narrated to us, from Thabit, from Anas, who attributed the hadith [to the Prophet], he said: "Indeed, fever is a furnace from the furnaces of Hell. Whoever is afflicted with any part of it, it is his share of the Fire."