Reference: I'laam al-Mu'aasireen bi-Fataawa Ibn 'Uthaymeen p425
Question: What is the ruling regarding wearing trousers?
Response: There is no harm in this, because the Prophet ﷺ said whoever is unable to find an izaar (a wrap around which covers the lower half of the body) then let him wear trousers [transmitted by al-Bukhaaree no.5804 and Muslim no.1178]. And whoever does not wear an izaar and [instead] needs to wear trousers, then let him wear those trousers which conform to the legislated requirement of covering the 'awrah (private parts) [and in so doing, not being tight].
Likewise, whoever is in need of that due to medical reasons, whereby some people say "if I do not wear these short trousers, this may cause severe sweat rash", then we say, there is no harm [in doing so] - wear [the trousers] and there is no [legislated] harm (penalty) upon you.
