as-salaam-u-alaikum my question is regarding wetness of underwear after watching something obscene. now the person wants to offer nimaaz, and he feels that the cloth has dried up. so,how can he purify the cloth,bec. he doesn't know the exact place where the wetness had occured

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

First of all, we inform you that you are obliged to repent to Allaah from watching those films because this is an act of disobedience and it is contrary to the Order of Allaah about lowering one’s gaze as Allaah says (what means): {Tell the believing men to lower their gaze (from looking at forbidden things) and protect their private parts. That is purer for them. Verily, Allaah is All-Aware of what they do.}[Quran 24:30] Looking at pornographic films causes psychological diseases and hardens one’s heart, and renders a person having no desire to what is lawful and encourages him to commit fornication or adultery and sins and to be careless about them, so you should fear Allaah and be shy of him because He is watching you; for more benefit about the obligation of lowering the gaze, please refer to Fataawa 85700 and 87190.

As regards the impurity that touches the clothes – whether it is Mathy, which is a white sticky fluid coming out when flirting with one’s wife or when thinking about sexual intercourse, or any other impurities – and its place is not known, then you should wash the entire cloth, and if you know the area of impurity, like being at the place facing one’s private parts, but you do not know exactly where, then you should wash from that area what you are sure or predominantly think that you have washed the place of impurity.

For more benefit, please refer to Fataawa 91596, 81587 and 82518. 

Allaah Knows best.