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 1280 f

Usama b. Zaid (Allah be pleased with him) reported that when Allah's Messenger (may peace jbe upod him) came to the valley where the rich (people of Mecca) used to get down. he got down. and urinated (and he did not mention about pouring water) ; he then called for water and performed a light ablution. I said:

Messenger of Allah, the prayer I Thereupon he said: Prayer awaits you ahead.
حَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، أَخْبَرَنَا وَكِيعٌ، حَدَّثَنَا سُفْيَانُ، عَنْ مُحَمَّدِ بْنِ عُقْبَةَ، عَنْ كُرَيْبٍ، عَنْ أُسَامَةَ بْنِ زَيْدٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم لَمَّا أَتَى النَّقْبَ الَّذِي يَنْزِلُهُ الأُمَرَاءُ نَزَلَ فَبَالَ - وَلَمْ يَقُلْ أَهْرَاقَ - ثُمَّ دَعَا بِوَضُوءٍ فَتَوَضَّأَ وُضُوءًا خَفِيفًا فَقُلْتُ يَا رَسُولَ اللَّهِ الصَّلاَةَ ‏.‏ فَقَالَ ‏ "‏ الصَّلاَةُ أَمَامَكَ ‏"‏ ‏.‏
Reference : Sahih Muslim 1280f
In-book reference : Book 15, Hadith 308
USC-MSA web (English) reference : Book 7, Hadith 2945
  (deprecated numbering scheme)