It was narrated that Ibn 'Abbas (رضي الله عنه) said:

They used to think that 'Umrah during the months of Hajj was one of the greatest evils on earth, and they would make Muharram Safar. They would say: when the backs of the camels have healed and the tracks of the pilgrims have become erased and Safar is over, 'Umrah becomes permissible for those who want to do 'Umrah. Then the Prophet (ﷺ) and his Companions came on the fourth (of Dhul Hijjah), reciting the Talbiyah for Hajj, and he told them to make it ‘Umrah. This was too hard for them, and they said: O Messenger of Allah, exiting ihram to what extent? He said: "Completely.”