Sahih Muslim Book 2, Hadith Number 572.

Abdullah b. Shihab al-Khaulani reported: I stayed in the house of ‘Aisha and had a wet dream (and perceived its effect on my garment), so (in the morning) I dipped both (the clothes) in water. This (act of mine) was watched by a maid-servant of ‘Aisha and she informed her. She (Hadrat ‘Aisha) sent me a message: What prompted you to act like this with your clothes? He (the narrator) said: I told that I saw in a dream what a sleeper sees. She said: Did you find (any mark of the fluid) on your clothes? I said: No. She said: Had you found anything you should have washed it. In-case I found that (semen) on the garment of the Messenger of Allah (may peace be upon him) dried up, I scraped it off with my nails.