Muwatta Imam Malik Book of Purity, Hadith 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."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّهُ سَمِعَهُ وَرَجُلٌ، يَسْأَلُهُ فَقَالَ إِنِّي لأَجِدُ الْبَلَلَ وَأَنَا أُصَلِّي، أَفَأَنْصَرِفُ فَقَالَ لَهُ سَعِيدٌ لَوْ سَالَ عَلَى فَخِذِي مَا انْصَرَفْتُ حَتَّى أَقْضِيَ صَلاَتِي .