كتاب الحج
15  
The Book of Pilgrimage
(26)
Chapter: It is permissible to exit Ihram if one is prevented from completing Hajj; It is permissible to perform Qiran and the pilgrim performing Qiran should perform just one Tawaf and one Sa'i
(26)
باب بَيَانِ جَوَازِ التَّحَلُّلِ بِالإِحْصَارِ وَجَوَازِ الْقِرَانِ ‏
Sahih Muslim 1230 c

Nafi' reported that Ibn Umar intended to go to Hajj (during the year) when Hajjaj attacked Ibn Zubair, and he narrated the account as (narrated above), and he used to say at the end of the hadith:

He who combines Hajj with Umra, for him one single circumambulation is sufficient, and he did not put off Ihram until he had completed both of them.
وَحَدَّثَنَاهُ ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، قَالَ أَرَادَ ابْنُ عُمَرَ الْحَجَّ حِينَ نَزَلَ الْحَجَّاجُ بِابْنِ الزُّبَيْرِ ‏.‏ وَاقْتَصَّ الْحَدِيثَ بِمِثْلِ هَذِهِ الْقِصَّةِ وَقَالَ فِي آخِرِ الْحَدِيثِ وَكَانَ يَقُولُ مَنْ جَمَعَ بَيْنَ الْحَجِّ وَالْعُمْرَةِ كَفَاهُ طَوَافٌ وَاحِدٌ وَلَمْ يَحِلَّ حَتَّى يَحِلَّ مِنْهُمَا جَمِيعًا ‏.‏
Reference : Sahih Muslim 1230c
In-book reference : Book 15, Hadith 199
USC-MSA web (English) reference : Book 7, Hadith 2840
  (deprecated numbering scheme)