Sayyiduna Ibn Abbas (may Allah be pleased with them both), attributing it to the Prophet (peace and blessings be upon him), said regarding the saying of Allah Almighty: "And if you are ill or on a journey" [Surah al-Nisa': 43]: When a man has a wound received in the path of Allah, or an ulcer, or smallpox, and he becomes junub and fears that if he takes a ritual bath he will die, then let him perform tayammum.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1066