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.

حَدَّثَنَا إِبْرَاهِيمُ بْنُ مُوسَى، أَخْبَرَنَا عَبْدُ اللَّهِ بْنُ وَهْبٍ، حَدَّثَنَا مُعَاوِيَةُ، - يَعْنِي ابْنَ صَالِحٍ - عَنِ الْعَلاَءِ بْنِ الْحَارِثِ، عَنْ حَرَامِ بْنِ حَكِيمٍ، عَنْ عَمِّهِ عَبْدِ اللَّهِ بْنِ سَعْدٍ الأَنْصَارِيِّ، قَالَ سَأَلْتُ رَسُولَ اللَّهِ ﷺ عَمَّا يُوجِبُ الْغُسْلَ وَعَنِ الْمَاءِ يَكُونُ بَعْدَ الْمَاءِ فَقَالَ
" ذَاكَ الْمَذْىُ وَكُلُّ فَحْلٍ يُمْذِي فَتَغْسِلُ مِنْ ذَلِكَ فَرْجَكَ وَأُنْثَيَيْكَ وَتَوَضَّأْ وُضُوءَكَ لِلصَّلاَةِ " .