Purification (Kitab Al-Taharah)
Dawud :: Book 1 : Hadith 211
Narrated Abdullah ibn Sa'd al-Ansari:
I asked the Apostle of Allah (peace_be_upon_him) as to what makes it necessary to take a bath and about the (prostatic) fluid that flows after taking a bath. He replied: that is called madhi (prostatic fluid). It flows from every male. You should wash your private parts and testicles because of it and perform ablution as you do for prayer.