Musnad Ahmad 25:436

'Affan narrated to us, Wuhayb narrated to us, 'Abdullah ibn Tawus narrated to us, from his father, from Ibn 'Abbas, who said: They used to consider performing 'Umrah during the months of Hajj to be the most wicked of sins on earth. They would make Muharram into Safar, and they would say: "When the sores on the camels' backs have healed, the tracks have been erased, and Safar has passed, 'Umrah becomes permissible for those who wish to perform it." When the Prophet, peace and blessings of Allah be upon him, and his companions arrived on the morning of the fourth [day of Dhu al-Hijjah], having entered into the state of Ihram for Hajj, he commanded them to change it into an 'Umrah. This seemed like a grave matter to them, so they said: "O Messenger of Allah, what kind of permissibility?" He said: "Complete permissibility."