It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you has a nosebleed during his prayer, let him leave the prayer, wash off the blood, then perform ablution anew and begin his prayer afresh."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11326