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
Sayyiduna Ibn Abbas (may Allah be pleased with them both) said regarding a man who becomes junub while he has a wound and fears that if he takes a ritual bath he will die: He should perform tayammum and pray.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1067
(a) Sayyiduna Ibn Abbas (may Allah be pleased with them both) said about a person with smallpox: He will perform tayammum.
(b) Sayyiduna Ibn Abbas (may Allah be pleased with them both) said about a person with smallpox and similar patients: When he becomes junub, he will perform tayammum with earth.
(c) It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the sick person has been given a concession to perform tayammum with earth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1068