Sahih al-Bukhari 178
Sahih al-Bukhari · Book 4: Ablutions (Wudu') · Whosoever considers not to repeat ablution except if something is discharged or passed from exit (front or back private parts)
Narrated `Ali:
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، قَالَ حَدَّثَنَا جَرِيرٌ ، عَنِ الأَعْمَشِ ، عَنْ مُنْذِرٍ أَبِي يَعْلَى الثَّوْرِيِّ ، عَنْ مُحَمَّدٍ ابْنِ الْحَنَفِيَّةِ ، قَالَ قَالَ عَلِيٌّ كُنْتُ رَجُلاً مَذَّاءً، فَاسْتَحْيَيْتُ أَنْ أَسْأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم فَأَمَرْتُ الْمِقْدَادَ بْنَ الأَسْوَدِ فَسَأَلَهُ فَقَالَ " فِيهِ الْوُضُوءُ " . وَرَوَاهُ شُعْبَةُ عَنِ الأَعْمَشِ .
I used to get emotional urethral discharges frequently and felt shy to ask Allah's Messenger (ﷺ) about it. So I requested Al-Miqdad bin Al-Aswad to ask (the Prophet (ﷺ) ) about it. Al-Miqdad asked him and he replied, "One has to perform ablution (after it)."