Musannaf Ibn Abi Shayba 1:2117

Abu Bakr narrated to us, he said: Abu Usama narrated to us, he said: Jarir ibn Hazim narrated to us, from Qays ibn Sa'd, from Sa'id ibn Jubayr and Mujahid, they both said regarding the sick person who experiences janaba (major ritual impurity) and fears for himself: He is in the position of the traveler who does not find water; he performs tayammum. And I asked 'Ata', and he said: There is no alternative to water, and it should be heated for him.