Sa'id bin al-Musayyib (may Allah have mercy on him) says that Sayyiduna Ali and Sayyiduna Uthman (may Allah be pleased with them both) met at a place called Usfan, and Sayyiduna Uthman (may Allah be pleased with him) used to forbid tamattu' Hajj. Sayyiduna Ali (may Allah be pleased with him) said to him: "Do you want to forbid something that the Messenger of Allah (peace and blessings be upon him) did?" He said: "Leave us alone." He said: "I cannot leave you alone." And he began to say the talbiyah for both Hajj and Umrah together.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8881