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 a

Ubaidullah b. 'Abdullah b. 'Umar reported on the authority of his father (Allah be pleased with them) that Allah's Messenger (ﷺ) combined the sunset and 'Isha', prayers at Muzdalifa and there was no prostration (i. e. any rak'ahs of Sunan or Nawafil prayers) in between them. He observed three rak'ahs of the sunset prayer and two rak'ahs of the 'Isha' prayer, and 'Abdullah (b. 'Umar) observed the prayers in this very manner (at Muzdalifa) until he met his Lord.

وَحَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى، أَخْبَرَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، أَنَّأَخْبَرَهُ أَنَّ أَبَاهُ قَالَ جَمَعَ رَسُولُ اللَّهِ صلى الله عليه وسلم بَيْنَ الْمَغْرِبِ وَالْعِشَاءِ بِجَمْعٍ لَيْسَ بَيْنَهُمَا سَجْدَةٌ وَصَلَّى الْمَغْرِبَ ثَلاَثَ رَكَعَاتٍ وَصَلَّى الْعِشَاءَ رَكْعَتَيْنِ ‏.‏ فَكَانَ عَبْدُ اللَّهِ يُصَلِّي بِجَمْعٍ كَذَلِكَ حَتَّى لَحِقَ بِاللَّهِ تَعَالَى ‏.‏
Reference : Sahih Muslim 1288a
In-book reference : Book 15, Hadith 315
USC-MSA web (English) reference : Book 7, Hadith 2952
  (deprecated numbering scheme)