Sahih Muslim: Hadith 667

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.