Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: the Messenger of Allah (peace and blessings be upon him) had Sayyidah Aishah (may Allah be pleased with her) perform Umrah in Dhu al-Hijjah only to put an end to the claim of the polytheists, for these Quraysh and those who followed their way used to say: When the wounds are gone, the marks have faded and the month of Safar has begun, then Umrah becomes lawful for the one who performs Umrah. Thus they used to regard Umrah as forbidden until Dhu al-Hijjah and Muharram had passed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8732