Mishkat al-Masabih 311
‘Ali said:
I asked the Prophet about prostatic fluid and he replied, “For prostatic fluid ablution is necessary, but for seminal emission washing.” 

Tirmidhi transmitted it.
وَعَن عَلِيٍّ قَالَ: سَأَلَتْ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَنْ الْمَذْيِ فَقَالَ: «مِنَ الْمَذْيِ الْوُضُوءُ وَمِنَ الْمَنِيِّ الْغسْل» . رَوَاهُ التِّرْمِذِيّ