Sahih Bukhari Volume 001, Book 004, Hadith Number 178.

Narated By 'Ali : I used to get emotional urethral discharges frequently and felt shy to ask Allah's Apostle about it. So I requested Al-Miqdad bin Al-Aswad to ask (the Prophet) about it. Al-Miqdad asked him and he replied, "On has to perform ablution (after it)."