It was narrated that Ibn 'Abbas said:"Ali, Al-Miqdad and 'Ammar were talking. 'Ali said: 'I am a man who emits a lot of Madhi but I am too shy to ask the Messenger of Allah (ﷺ) about that because of his daughter's position with me, so let one of you ask him.' He told me that one of them - but I forgot who - asked him, and the Prophet (ﷺ) said: 'That is Madhi. If any one of you notices that, let him wash it off himself and perform Wudu' as for prayer or similar to the Wudu' of prayer.'"
أَخْبَرَنَا عَلِيُّ بْنُ مَيْمُونٍ، قَالَ حَدَّثَنَا مَخْلَدُ بْنُ يَزِيدَ، عَنِ ابْنِ جُرَيْجٍ، عَنْ عَطَاءٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ تَذَاكَرَ عَلِيٌّ وَالْمِقْدَادُ وَعَمَّارٌ فَقَالَ عَلِيٌّ إِنِّي امْرَؤٌ مَذَّاءٌ وَإِنِّي أَسْتَحِي أَنْ أَسْأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم لِمَكَانِ ابْنَتِهِ مِنِّي فَيَسْأَلُهُ أَحَدُكُمَا فَذَكَرَ لِي أَنَّ أَحَدَهُمَا وَنَسِيتُهُ سَأَلَهُ فَقَالَ النَّبِيُّ صلى الله عليه وسلم " ذَاكَ الْمَذْىُ إِذَا وَجَدَهُ أَحَدُكُمْ فَلْيَغْسِلْ ذَلِكَ مِنْهُ وَلْيَتَوَضَّأْ وُضُوءَهُ لِلصَّلاَةِ أَوْ كَوُضُوءِ الصَّلاَةِ " .
Nesai
The Book of Ghusl and Tayammum • Hadith 435