Sunan An-Nasai, Chapter: The Book of Ghusl and Tayammum

"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 (saw) about that because if 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 (saw) 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.'" (Sahih)