سنن النسائي

It was narrated that Ibn Abbas said: "They used to think that performing 'Umrah during the months of Hajj was one of the worst of evil actions on Earth, and they used to call Muharram 'Safar,' and say: 'When the sore on the backs of the camels have healed and when their hair grows back and when Safar is over' - or he said: 'When Safar beings - then 'Umrah becomes permissible for whoever wants to do it.' Then the Prophet (ﷺ) and his companions came on the morning of the fourth of Dhul-Hijjah, reciting the Talbiyah for Hajj, He told them to make it 'Umrah, and they found it too difficult to do that. They said: 'O Messenger of Allah, to what degree should we exit Ihram?' He said: 'Completely.'"