Muwatta Malik 84

Yahya related to me from Malik from Abu'n Nadr, the mawla of Abdullah ibn Ubaydullah, from Sulayman ibn Yasar from alMiqdad ibn al- Aswad that Ali ibn Abi Talib told him to ask the Messenger of Allah, may Allah bless him and grant him peace, what a man should do, who, when close to his wife, had a flow of prostatic fluid. Ali explained that the daughter of the Messenger of Allah, may Allah bless him and grant him peace, was living with him then and he was too shy to ask for himself. Al-Miqdad said, "I asked the Messenger of Allah, may Allah bless him and grant him peace, about it, and he said, 'When you find that, wash your genitals with water and do wudu as for prayer