It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that a man came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah, I suffer from a fistula (a wound that discharges continuously), and it keeps flowing from me. So the Prophet (peace and blessings be upon him) said: "When you have performed ablution from your head to your feet, then another ablution is not required of you."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11156