Sunan an-Nasa'i 193
سنن النسائي#193Sahihأَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، وَعَلِيُّ بْنُ حُجْرٍ، - وَاللَّفْظُ لِقُتَيْبَةَ - قَالَ حَدَّثَنَا عَبِيدَةُ بْنُ حُمَيْدٍ، عَنِ الرُّكَيْنِ بْنِ الرَّبِيعِ، عَنْ حُصَيْنِ بْنِ قَبِيصَةَ، عَنْ عَلِيٍّ، - رضى الله عنه - قَالَ كُنْتُ رَجُلاً مَذَّاءً فَقَالَ لِي رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا رَأَيْتَ الْمَذْىَ فَاغْسِلْ ذَكَرَكَ وَتَوَضَّأْ وُضُوءَكَ لِلصَّلاَةِ وَإِذَا فَضَخْتَ الْمَاءَ فَاغْتَسِلْ " .
It was narrated that 'Ali said:"I was one who had a lot of prostatic discharge, and the Messenger of Allah (ﷺ) said to me: 'If you see Madhi (prostatic fluid) then wash your penis and perform Wudu' like that for Salah, but if you ejaculate semen, then perform Ghusl
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 1:194