Muwatta Malik 87

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 ‏.‏

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

Reference

Muwatta Malik 87

In-book

Book 2, Hadith 56