Musnad Ahmad 1153:922

'Affan and Bahz narrated to us, they said: Shu'bah narrated to us, he said: Al-Hakam informed me, from Ibrahim, from Hammam ibn al-Harith, that he was staying with 'Aishah. Bahz said: A man from the Nakha' tribe was staying with 'Aishah, and he had a wet dream. A slave girl of 'Aishah saw him washing the trace of the seminal fluid from his garment, or washing his garment—Bahz said: This is how Shu'bah said it. She said: "I have seen myself doing nothing more than rubbing it off the garment of the Messenger of Allah, may Allah bless him and grant him peace."