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 d

Ibn 'Abbas (Allah be pleased with them) reported:

Allah's Messenger (ﷺ) came along with his Companions when four days had passed out of ten days (of Dhu'l-Hijja) and they were pronouncing Talbiya for Hajj, and he (the Holy Prophet) commanded them to change (this Ihram) into that of 'Umra.
وَحَدَّثَنَا هَارُونُ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا مُحَمَّدُ بْنُ الْفَضْلِ السَّدُوسِيُّ، حَدَّثَنَا وُهَيْبٌ، أَخْبَرَنَا أَيُّوبُ، عَنْ أَبِي الْعَالِيَةِ الْبَرَّاءِ، عَنِ ابْنِ عَبَّاسٍ، - رضى الله عنهما - قَالَ قَدِمَ النَّبِيُّ صلى الله عليه وسلم وَأَصْحَابُهُ لأَرْبَعٍ خَلَوْنَ مِنَ الْعَشْرِ وَهُمْ يُلَبُّونَ بِالْحَجِّ فَأَمَرَهُمْ أَنْ يَجْعَلُوهَا عُمْرَةً ‏.‏
Reference : Sahih Muslim 1240d
In-book reference : Book 15, Hadith 220
USC-MSA web (English) reference : Book 7, Hadith 2861
  (deprecated numbering scheme)