Marwan bin al-Hakam (may Allah have mercy on him) says: I heard Uthman and Ali (may Allah be pleased with them both) between Makkah and Madinah. Uthman (may Allah be pleased with him) was forbidding mut'ah, that Hajj and Umrah should be combined. When Ali (may Allah be pleased with him) saw this, he began to say the talbiyah for both Hajj and Umrah. Uthman (may Allah be pleased with him) said: I forbid something and you do it? He said: I will not abandon the Sunnah of the Messenger of Allah (peace and blessings be upon him) because of the word of any person.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8880