Narrated Ibn 'Abbas:

The people used to consider the performance of 'Umra in the months of Hajj an evil deed on the earth, and they used to call the month of Muharram as Safar and used to say, "When (the wounds over) the backs (of the camels) have healed and the foot-marks (of the camels) have vanished (after coming from Hajj), then 'Umra becomes legal for the one who wants to perform 'Umra." Allah's Apostle and his companions reached Mecca assuming Ihram for Hajj on the fourth of Dhul-Hijja. The Prophet ordered his companions to perform 'Umra (with that lhram instead of Hajj). They asked, "O Allah's Apostle! What kind of finishing of Ihram?" The Prophet ﷺ said, "Finish the Ihram completely.'

حَدَّثَنَا مُسْلِمٌ، حَدَّثَنَا وُهَيْبٌ، حَدَّثَنَا ابْنُ طَاوُسٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ كَانُوا يَرَوْنَ أَنَّ الْعُمْرَةَ فِي أَشْهُرِ الْحَجِّ مِنَ الْفُجُورِ فِي الأَرْضِ، وَكَانُوا يُسَمُّونَ الْمُحَرَّمَ صَفَرًا وَيَقُولُونَ إِذَا بَرَا الدَّبَرْ، وَعَفَا الأَثَرْ، حَلَّتِ الْعُمْرَةُ لِمَنِ اعْتَمَرْ. قَالَ فَقَدِمَ رَسُولُ اللَّهِ ﷺ وَأَصْحَابُهُ رَابِعَةً مُهِلِّينَ بِالْحَجِّ وَأَمَرَهُمُ النَّبِيُّ ﷺ أَنْ يَجْعَلُوهَا عُمْرَةً. قَالُوا يَا رَسُولَ اللَّهِ، أَىُّ الْحِلِّ قَالَ " الْحِلُّ كُلُّهُ ".