Sunan Abi Dawud

Narrated Abdullah ibn Sa'd al-Ansari: I asked the Messenger of Allah (ﷺ) 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