Sahih Muslim 1240 a
Ibn Abbas (Allah be pleased with them) reported that they (the Arabs of pre-Islamic days) looked upon Umra during the months of Hajj as the greatest of sins on the earth. So they intercalated the month of Muharram for Safar and said:
When the backs of their camels would become all right and traces (if the pilgrims) would be effaced (from the paths) and the month of Safar would be over, then Umra would be permissible for one who wants to perform it. When Allah's Apostle (ﷺ) and his Companions came in the state of Ihram for performing Hajj on the fourth (of Dhu'l-Hijja) he (Allah's Apostle) commanded them to change their state of Ihram (from Hajj) to that of 'Umra. It was something inconceivable for them. So they said: Messenger of Allah, is it a complete freedom (of the obligation) of Ihram? Thereupon he said: It is a complete freedom (from Ihram).
وَحَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ، حَدَّثَنَا بَهْزٌ، حَدَّثَنَا وُهَيْبٌ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ طَاوُسٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ، - رضى الله عنهما - قَالَ كَانُوا يَرَوْنَ أَنَّ الْعُمْرَةَ فِي أَشْهُرِ الْحَجِّ مِنْ أَفْجَرِ الْفُجُورِ فِي الأَرْضِ وَيَجْعَلُونَ الْمُحَرَّمَ صَفَرً وَيَقُولُونَ إِذَا بَرَأَ الدَّبَرْ وَعَفَا الأَثَرْ وَانْسَلَخَ صَفَرْ حَلَّتِ الْعُمْرَةُ لِمَنِ اعْتَمَرْ ‏.‏ فَقَدِمَ النَّبِيُّ صلى الله عليه وسلم وَأَصْحَابُهُ صَبِيحَةَ رَابِعَةٍ مُهِلِّينَ بِالْحَجِّ فَأَمَرَهُمْ أَنْ يَجْعَلُوهَا عُمْرَةً فَتَعَاظَمَ ذَلِكَ عِنْدَهُمْ فَقَالُوا يَا رَسُولَ اللَّهِ أَىُّ الْحِلِّ قَالَ ‏
"‏ الْحِلُّ كُلُّهُ ‏"‏ ‏.‏