Sahih Bukhari Volume 001, Book 004, Hadith Number 232.

Narated By 'Amr bin Maimun : I heard Sulaiman bin Yasar talking about the clothes soiled with semen. He said that 'Aisha had said, "I used to wash it off the clothes of Allah's Apostle and he would go for the prayers while water spots were still visible on them.