If using the water would make him ill, then he falls under the general meaning of Allah’s saying: “If you are ill or on a journey…” [al-Ma'idah 5:6].
For example, if he has sores on the limbs washed in ablution, or if, when a full ritual bath is required, he has sores all over his body and fears bodily harm, then he may perform dry ablution.
The same applies if he fears the cold: he should heat the water, but if he cannot find anything with which to heat it, he performs dry ablution, because he fears bodily harm. Allah, Most High, says: “Do not kill yourselves, for Allah is ever Merciful to you” [al-Nisa' 4:29]. Amr ibn al-As, may Allah be pleased with him, cited this verse as evidence for the permissibility of dry ablution in cold weather when a full ritual bath is required (1).
His statement, “or harm to his body from seeking it,” means that he fears bodily harm from seeking water because it is somewhat distant, or because of the intense cold of the weather; in that case, he performs dry ablution.
The evidence for this is His saying, Exalted is He: “Do not kill yourselves” [al-Nisa' 4:29]; and His saying: “Do not throw yourselves into destruction with your own hands” [al-Baqarah 2:195]; and His saying: “He has not placed any hardship upon you in religion” [al-Hajj 22:78]. Fear of harm is a hardship. The Prophet, peace and blessings be upon him, said: “There should be neither harm nor reciprocating harm” (2).