Sahih Muslim
The Book of Menstruation (Kitab Al-Haid)
Book Number: 003 / Hadith Number: 594
'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.