Sayyiduna Ali (may Allah be pleased with him) narrates: "When a person is leading the people in prayer and he feels rumbling in his stomach, or his nose starts to bleed, or he vomits, then he should place a cloth on his nose and take the hand of one of those present and bring him forward."
Hadith Referenceسنن الدارقطني / كتاب الطهارة / 576