Book 2, Number 2.15.58:

Yahya related to me from Malik from Yahya ibn Said that he was listening to Said ibn al-Musayyab and a man questioned him saying, "I discover a liquid when I am praying. Should I leave?"

Said ibn al-Musayyab said to him, "Even if it were to flow on my leg I would not leave until I had finished the prayer."