Sahih Muslim Book 003, Hadith Number 0594.
Chapter : On al-Madhi.
'Ali reported: I felt shy of asking about prostatic fluid from the Apostle (may peace be upon him) because of Fatimah. I, therefore, asked al-Miqdad (to ask on my behalf) and he asked. He (the Holy Prophet) said: Ablution is obligatory in such a case.