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 b

Sa'id b. Jubair reported that he observed the sunset and 'Isha' prayers at Muzdalifa with (one) iqama. He narrated on the authority of Ibn 'Umar (Allah be pleased with them) that he observed prayers like this and Ibn 'Umar (Allah be pleased with them) narrated that Allah's Apostle (ﷺ) did like this.

حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ مَهْدِيٍّ، حَدَّثَنَا شُعْبَةُ، عَنِ الْحَكَمِ، وَسَلَمَةَ بْنِ كُهَيْلٍ عَنْ سَعِيدِ بْنِ جُبَيْرٍ، أَنَّهُ صَلَّى الْمَغْرِبَ بِجَمْعٍ وَالْعِشَاءَ بِإِقَامَةٍ ثُمَّ حَدَّثَ عَنِ ابْنِ عُمَرَ أَنَّهُ صَلَّى مِثْلَ ذَلِكَ وَحَدَّثَ ابْنُ عُمَرَ أَنَّ النَّبِيَّ صلى الله عليه وسلم صَنَعَ مِثْلَ ذَلِكَ ‏.‏
Reference : Sahih Muslim 1288b
In-book reference : Book 15, Hadith 316
USC-MSA web (English) reference : Book 7, Hadith 2953
  (deprecated numbering scheme)