Marwan bin al-Hakam said: I found Sayyiduna Ali and Sayyiduna Uthman (may Allah be pleased with them both) in Makkah, and Sayyiduna Uthman (may Allah be pleased with him) used to forbid combining Hajj and Umrah, and also Hajj al-Tamattu'. When Sayyiduna Ali (may Allah be pleased with him) learned of this, he made the talbiyah for Hajj al-Qiran and said: 'Labbayka bi-'umratin wa hajjatin ma'an' (Here I am, for Umrah and Hajj together). Then Sayyiduna Uthman (may Allah be pleased with him) said: Do you know that I forbid the people from something and then you do that very thing? Sayyiduna Ali (may Allah be pleased with him) said: I will not abandon the Sunnah of the Messenger of Allah (peace and blessings be upon him) for the sake of anyone's word.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8773