(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