كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(1)
Chapter: The travelers’ prayer and shortening it
(1)
باب صَلاَةِ الْمُسَافِرِينَ وَقَصْرِهَا ‏‏
Sahih Muslim 685 a

'A'isha, the wife of the Messenger of Allah (ﷺ), reported:

The prayer was prescribed as two rak'ahs, two rak'ahs both in journey and at the place of residence. The prayer while travelling remained as it was (originally prescribed), but an addition was made in the prayer (observed) at the place of residence.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ صَالِحِ بْنِ كَيْسَانَ، عَنْ عُرْوَةَ بْنِ الزُّبَيْرِ، عَنْ عَائِشَةَ، زَوْجِ النَّبِيِّ صلى الله عليه وسلم أَنَّهَا قَالَتْ فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ رَكْعَتَيْنِ فِي الْحَضَرِ وَالسَّفَرِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَزِيدَ فِي صَلاَةِ الْحَضَرِ ‏.‏
Reference : Sahih Muslim 685a
In-book reference : Book 6, Hadith 1
USC-MSA web (English) reference : Book 4, Hadith 1458
  (deprecated numbering scheme)