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 e

Ibn Abbas (Allah be pleased with them) reported that Allah's Messenger (ﷺ) observed the morning prayer at Dhu Tawa (a valley near Mecca) and arrived (in Mecca) when four days of Dhul-Hijja had passed and he commanded his Companions that they should change their Ihram (of Hajj) to that of Umra, except those who had brought sacrificial animals with them.

وَحَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنْ أَيُّوبَ، عَنْ أَبِي الْعَالِيَةِ، عَنِ ابْنِ عَبَّاسٍ، - رضى الله عنهما - قَالَ صَلَّى رَسُولُ اللَّهِ صلى الله عليه وسلم الصُّبْحَ بِذِي طَوًى وَقَدِمَ لأَرْبَعٍ مَضَيْنَ مِنْ ذِي الْحِجَّةِ وَأَمَرَ أَصْحَابَهُ أَنْ يُحَوِّلُوا إِحْرَامَهُمْ بِعُمْرَةٍ إِلاَّ مَنْ كَانَ مَعَهُ الْهَدْىُ ‏.‏
Reference : Sahih Muslim 1240e
In-book reference : Book 15, Hadith 221
USC-MSA web (English) reference : Book 7, Hadith 2862
  (deprecated numbering scheme)