Washing cloths after a wet dream or getting conatminated with sperm by some other means

Question

Can I just wash the part of cloth which is contaminated with sperm after a wet dream and then pray or do I need to wash the whole cloth?

Answer 1

Alqama and Aswad reported:

أَنَّ رَجُلاً، نَزَلَ بِعَائِشَةَ فَأَصْبَحَ يَغْسِلُ ثَوْبَهُ فَقَالَتْ عَائِشَةُ إِنَّمَا كَانَ يُجْزِئُكَ إِنْ رَأَيْتَهُ أَنْ تَغْسِلَ مَكَانَهُ فَإِنْ لَمْ تَرَ نَضَحْتَ حَوْلَهُ وَلَقَدْ رَأَيْتُنِي أَفْرُكُهُ مِنْ ثَوْبِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَرْكًا فَيُصَلِّي فِيهِ

A person stayed in the house of A'isha and in the morning began to wash his garment. A'isha said: In case you saw it (i. e. drop of semen), it would have served the purpose (of purifying the garment) if you had simply washed that spot; and in case you did not see it, it would have been enough to sprinkle water around it, for when I saw that on the garment of the Messenger of Allah (ﷺ). I simply scraped it off and he offered prayer, while putting that on.

^{- Sahih Muslim 566}

According to the hadith, you just have to wash the part that has semen.

Answer 2

If you consider sperm as najis then washing it out is sufficient as you don't need to wash the whole piece of cloth this is the view of abu Hanifa and one opinion of imam Ahmad while imam Malik said one must wash it off, apparently without stating it is najis.
Note that washing something off (or using water) usually is done for cleaning najasa.

Else there's nothing to do, but one could rub or scrap its -dried- trace off. This is the view of imam a-Shafi'i and the most known opinion held by imam Ahmad.

Both interpretations are based on the hadith of 'Aisha which you may find in Sahih Muslim, Sunan abi Dawod and Jami' at-Tirmdihi (who said that a-Thawri, a-Shafi'i, Ahmad and Ishaaq held the opinion that scraping off the many -semen- is sufficient) which states:

...it would have served the purpose (of purifying the garment) if you had simply washed that spot ... when I saw that on the garment of the Messenger of Allah (ﷺ). I simply scraped it off and he offered prayer, while putting that on.

See also fatwa islamqa #170012.

It is nevertheless recommended to either remove the traces of sperms by washing them off if they are still wet or can be seen on clothes even after the attempt of scrapping them off or scrapping them off when they are dry.