Narrated Abdullah ibn Abbas: By Allah, the Messenger of Allah (ﷺ) did not make Aisha perform umrah during Dhul-Hijjah but to discontinue the practice of the idolaters (in Arabia before Islam), for this clan of Quraysh and those who followed them used to say: When the fur of the camel abounds, and the wounds on the back of the camels are recovered and the month of Safar begins, umrah becomes lawful for one who performs umrah. They considered performing umrah unlawful till the months of Dhul-Hijjah and al-Muharram passed away.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا هَنَّادُ بْنُ السَّرِيِّ، عَنِ ابْنِ أَبِي زَائِدَةَ، حَدَّثَنَا ابْنُ جُرَيْجٍ، وَمُحَمَّدُ بْنُ إِسْحَاقَ، عَنْ عَبْدِ اللَّهِ بْنِ طَاوُسٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ وَاللَّهِ مَا أَعْمَرَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَائِشَةَ فِي ذِي الْحِجَّةِ إِلاَّ لِيَقْطَعَ بِذَلِكَ أَمْرَ أَهْلِ الشِّرْكِ فَإِنَّ هَذَا الْحَىَّ مِنْ قُرَيْشٍ وَمَنْ دَانَ دِينَهُمْ كَانُوا يَقُولُونَ إِذَا عَفَا الْوَبَرْ وَبَرَأَ الدَّبَرْ وَدَخَلَ صَفَرْ فَقَدْ حَلَّتِ الْعُمْرَةُ لِمَنِ اعْتَمَرْ . فَكَانُوا يُحَرِّمُونَ الْعُمْرَةَ حَتَّى يَنْسَلِخَ ذُو الْحِجَّةِ وَالْمُحَرَّمُ .
sunnah.com