كتاب الحج
15  
The Book of Pilgrimage
(47)
Chapter: Departing from 'Arafat to Al-Muzdalifah. It is recommended to pray Maghrib and 'Isha together in Al-Muzdalifah on this night
(47)
باب الإِفَاضَةِ مِنْ عَرَفَاتٍ إِلَى الْمُزْدَلِفَةِ وَاسْتِحْبَابِ صَلاَتَىِ الْمَغْرِبِ وَالْعِشَاءِ جَمْعًا بِالْمُزْدَلِفَةِ فِي هَذِهِ اللَّيْلَةِ
Sahih Muslim 1288 c

Shu'ba reported this hadith with the same chain of transmitters and said:

He (the Holy Prophet) observed the two prayers (together) with one iqama.
وَحَدَّثَنِيهِ زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا شُعْبَةُ، بِهَذَا الإِسْنَادِ وَقَالَ صَلاَّهُمَا بِإِقَامَةٍ وَاحِدَةٍ ‏.‏
Reference : Sahih Muslim 1288c
In-book reference : Book 15, Hadith 317
USC-MSA web (English) reference : Book 7, Hadith 2953
  (deprecated numbering scheme)