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 1286 a

Ibn Abbas (Allah be pleased with him) reported that Allah's Messenger (may peace be upon, him) came back from 'Arafa and Usama (Allah be pleased with him) was seated behind him. Usama said that he (the Holy Prophet) continued the journey in this very state until he came to Muzdalifa.

حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ، أَخْبَرَنَا عَبْدُ الْمَلِكِ بْنُ أَبِي سُلَيْمَانَ، عَنْ عَطَاءٍ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أَفَاضَ مِنْ عَرَفَةَ وَأُسَامَةُ رِدْفُهُ قَالَ أُسَامَةُ فَمَازَالَ يَسِيرُ عَلَى هَيْئَتِهِ حَتَّى أَتَى جَمْعًا ‏.‏
Reference : Sahih Muslim 1286a
In-book reference : Book 15, Hadith 310
USC-MSA web (English) reference : Book 7, Hadith 2947
  (deprecated numbering scheme)