ʿAli and ʿUthman differed regarding Hajj-at-Tamattuʿ while they were at 'Usfan (a familiar place near Mecca). ʿAli said, "I see you want to forbid people to do a thing that the Prophet ﷺ did?" When ʿAli saw that, he assumed Ihram for both Hajj and ʿUmra.
اخْتَلَفَ عَلِيٌّ وَعُثْمَانُ ؓ وَهُمَا بِعُسْفَانَ فِي الْمُتْعَةِ فَقَالَ عَلِيٌّ مَا تُرِيدُ إِلاَّ أَنْ تَنْهَى عَنْ أَمْرٍ فَعَلَهُ النَّبِيُّ ﷺ فَلَمَّا رَأَى ذَلِكَ عَلِيٌّ أَهَلَّ بِهِمَا جَمِيعًا
What is your opinion about a matter which the Messenger of Allah ﷺ did but you forbid it? Thereupon Uthman said: You leave us alone, whereupon he ('Ali) said: I cannot leave you alone. When 'Ali saw this, he put on Ihram for both of them together (both for Hajj and 'Umra).
اجْتَمَعَ عَلِيٌّ وَعُثْمَانُ ؓ بِعُسْفَانَ فَكَانَ عُثْمَانُ يَنْهَى عَنِ الْمُتْعَةِ أَوِ الْعُمْرَةِ فَقَالَ عَلِيٌّ مَا تُرِيدُ إِلَى أَمْرٍ فَعَلَهُ رَسُولُ اللَّهِ ﷺ تَنْهَى عَنْهُ فَقَالَ عُثْمَانُ دَعْنَا مِنْكَ فَقَالَ إِنِّي لاَ أَسْتَطِيعُ أَنْ أَدَعَكَ فَلَمَّا أَنْ رَأَى عَلِيٌّ ذَلِكَ أَهَلَّ بِهِمَا جَمِيعًا