Sahih Muslim Book 2, Hadith Number 566.

Chapter : Washing away of the semen from the garment and its scraping.

Alqama and Aswad reported: A person stayed in the house of ‘Aisha and in the morning began to wash his garment. ‘Aisha 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 (may peace be upon him). I simply scraped it off and he offered prayer, while putting that on.