كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(5)
Chapter: It is permissible to combine two prayers when traveling
(5)
باب جَوَازِ الْجَمْعِ بَيْنَ الصَّلاَتَيْنِ فِي السَّفَرِ ‏
Sahih Muslim 704 a

Anas b. Malik reported:

When the Messenger of Allah (ﷺ) set out on a journey before the sun declined (from the meridian), he delayed the noon prayer till the afternoon prayer, and then dismounted (his ride) and combined them (noon and afternoon prayers), but if the sun had declined before his setting out on a journey, he observed the noon prayer and then mounted (the ride).
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا الْمُفَضَّلُ، - يَعْنِي ابْنَ فَضَالَةَ - عَنْ عُقَيْلٍ، عَنِ ابْنِ شِهَابٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا ارْتَحَلَ قَبْلَ أَنْ تَزِيغَ الشَّمْسُ أَخَّرَ الظُّهْرَ إِلَى وَقْتِ الْعَصْرِ ثُمَّ نَزَلَ فَجَمَعَ بَيْنَهُمَا فَإِنْ زَاغَتِ الشَّمْسُ قَبْلَ أَنْ يَرْتَحِلَ صَلَّى الظُّهْرَ ثُمَّ رَكِبَ ‏.‏
Reference : Sahih Muslim 704a
In-book reference : Book 6, Hadith 56
USC-MSA web (English) reference : Book 4, Hadith 1512
  (deprecated numbering scheme)