Sunan an-Nasa'i 2813
It was narrated that Ibn Abbas said:
"They used to think that performing 'Umrah during the months of Hajj was one of the worst of evil actions on Earth, and they used to call Muharram 'Safar,' and say: 'When the sore on the backs of the camels have healed and when their hair grows back and when Safar is over' - or he said: 'When Safar beings - then 'Umrah becomes permissible for whoever wants to do it.' Then the Prophet and his companions came on the morning of the fourth of Dhul-Hijjah, reciting the Talbiyah for Hajj, He told them to make it 'Umrah, and they found it too difficult to do that. They said: 'O Messenger of Allah, to what degree should we exit Ihram?' He said: 'Completely.'"
أَخْبَرَنَا عَبْدُ الأَعْلَى بْنُ وَاصِلِ بْنِ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ وُهَيْبِ بْنِ خَالِدٍ، قَالَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ طَاوُسٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ كَانُوا يُرَوْنَ أَنَّ الْعُمْرَةَ، فِي أَشْهُرِ الْحَجِّ مِنْ أَفْجَرِ الْفُجُورِ فِي الأَرْضِ وَيَجْعَلُونَ الْمُحَرَّمَ صَفَرَ وَيَقُولُونَ إِذَا بَرَأَ الدَّبَرْ وَعَفَا الْوَبَرْ وَانْسَلَخَ صَفَرْ - أَوْ قَالَ دَخَلَ صَفَرْ - فَقَدْ 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 ‏"‏ ‏.‏