كتاب الإمارة
33  
The Book on Government
(10)
Chapter: The obligation of fulfilling oaths of allegiance is owed to the first of two Caliphs
(10)
باب الْوَفَاءِ بِبَيْعَةِ الْخُلَفَاءِ الأَوَّلِ فَالأَوَّلِ ‏‏
Sahih Muslim 1842 b

The same tradition has been transmitted by a different chain of narrators.

Reference : Sahih Muslim 1842b
In-book reference : Book 33, Hadith 72
USC-MSA web (English) reference : Book 20, Hadith 4544
  (deprecated numbering scheme)