كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(48)
Chapter: The clarification that the Qur’an was revealed in seven ahruf (modes of recitation), and clarifying its meaning
(48)
باب بَيَانِ أَنَّ الْقُرْآنَ عَلَى سَبْعَةِ أَحْرُفٍ وَبَيَانِ مَعْنَاهُ ‏
Sahih Muslim 821 b

This hadith has been narrated by Shu'ba with the same chain of transmitters.

وَحَدَّثَنَاهُ عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، بِهَذَا الإِسْنَادِ مِثْلَهُ ‏.‏
Reference : Sahih Muslim 821b
In-book reference : Book 6, Hadith 335
USC-MSA web (English) reference : Book 4, Hadith 1790
  (deprecated numbering scheme)