Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: The people of Jahiliyyah regarded performing Umrah in the months of Hajj as one of the gravest sins on the face of the earth, and they used to say: When the wounds on the camels' backs have healed, the marks have faded and the month of Safar has ended, then Umrah becomes lawful for the one who performs Umrah. They used to call Muharram Safar. the Prophet (peace and blessings be upon him) and his Companions (may Allah be pleased with them) arrived on the fourth of Dhu al-Hijjah reciting the talbiyah for Hajj, and the Messenger of Allah (peace and blessings be upon him) commanded them: "Make it an Umrah." This weighed heavily on them. They said: How will we come out of ihram after the Umrah? He (peace and blessings be upon him) said: "Complete release, that is, everything that was forbidden to them because of ihram becomes permissible."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8733