ذِكْرُ الْإِبَاحَةِ لِلْمَرْءِ أَنْ يُصَلِّيَ فِي الثَّوْبِ الَّذِي أَصَابَهُ الْمَنِيُّ، وَإِنْ لَمْ يَغْسِلْهُ

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. (Using translation from Muslim 288a)  

أَنَّ رَجُلًا نَزَلَ بِعَائِشَةَ أُمِّ الْمُؤْمِنِينَ فَأَصْبَحَ يَغْسِلُ ثَوْبَهُ فَقَالَتْ عَائِشَةُ «إِنَّمَا كَانَ يُجْزِيكَ إِنْ رَأَيْتَهُ أَنْ تَغْسِلَ مَكَانَهُ وَإِنْ لَمْ تَرَهُ نَضَحْتَ حَوْلَهُ لَقَدْ رَأَيْتُنِي أَفْرُكُهُ مِنْ ثَوْبِ رَسُولِ اللَّهِ ﷺ فَرْكًا فَيُصَلِّي فِيهِ»