Abu Razin says: I prayed behind Ali bin Abi Talib (may Allah be pleased with him). His nose began to bleed, so he stepped back, took a man by the hand and put him forward; he led the prayer, and Ali (may Allah be pleased with him) went away.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5258