كتاب الحج
15  
The Book of Pilgrimage
(31)
Chapter: It is permissible to perform 'Umrah during the months of Hajj
(31)
باب جَوَازِ الْعُمْرَةِ فِي أَشْهُرِ الْحَجِّ ‏
Sahih Muslim 1240 b

Ibn 'Abbas (Allah be pleased with them) 'is reported to have said:

The Messenger of Allah (ﷺ) put on Ihigm for Hajj. When four days of Dhu'l-Hijja were over, he led the dawn prayer, and when the prayer was complete, he said: He who wants to change it to Umra may do so.
حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، عَنْ أَيُّوبَ، عَنْ أَبِي الْعَالِيَةِ، الْبَرَّاءِ أَنَّهُ سَمِعَ ابْنَ عَبَّاسٍ، - رضى الله عنهما - يَقُولُ أَهَلَّ رَسُولُ اللَّهِ صلى الله عليه وسلم بِالْحَجِّ فَقَدِمَ لأَرْبَعٍ مَضَيْنَ مِنْ ذِي الْحِجَّةِ فَصَلَّى الصُّبْحَ وَقَالَ لَمَّا صَلَّى الصُّبْحَ ‏ "‏ مَنْ شَاءَ أَنْ يَجْعَلَهَا عُمْرَةً فَلْيَجْعَلْهَا عُمْرَةً ‏"‏ ‏.‏
Reference : Sahih Muslim 1240b
In-book reference : Book 15, Hadith 218
USC-MSA web (English) reference : Book 7, Hadith 2859
  (deprecated numbering scheme)