Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: By Allah! 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, by it, to the saying of the people of shirk (polytheism). For this tribe of Quraysh, and those who followed their religion, used to say: "When the hair (of the camels) has grown back, the sores have healed, and Safar has begun, then Umrah becomes lawful for whoever wishes to perform it." So they would consider Umrah forbidden until Dhu al-Hijjah and Muharram had passed. By Allah! The Messenger of Allah (peace and blessings be upon him) had Sayyidah Aishah (may Allah be pleased with her) perform Umrah only to put an end to this saying of theirs.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10861