Al-Hasan (may Allah have mercy on him) says that Sayyiduna Ali (may Allah be pleased with him) led us in prayer for twenty nights in the time of Sayyiduna Uthman bin Affan (may Allah be pleased with him), then he stopped. Some said: He has set aside time for himself. Then Abu Halimah Mu'adh (may Allah have mercy on him) led them, and he would recite the qunut.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4632