Sunan an Nasai
194
Sahih+2

It was narrated that 'Ali said:"I was one who had a lot of prostatic discharge, so I asked the Prophet (ﷺ) and he said: 'If you see Madhi (prostatic fluid) then perform Wudu' and wash your penis, but if you see semen ejaculated, then perform Ghusl

SahihAbu Ghuddah
SahihAl-Albani