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

'A'isha reported:

The prayer was prescribed as consisting of two rak'ahs, the prayer in travelling remained the same, but the prayer at the place of residence was completed. (Zuhri said he asked 'Urwa why 'A'isha said prayer in the complete form during journey, and he replied that she interpreted the matter herself as 'Uthman did.)
وَحَدَّثَنِي عَلِيُّ بْنُ خَشْرَمٍ، أَخْبَرَنَا ابْنُ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ الصَّلاَةَ، أَوَّلَ مَا فُرِضَتْ رَكْعَتَيْنِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَأُتِمَّتْ صَلاَةُ الْحَضَرِ ‏.‏ قَالَ الزُّهْرِيُّ فَقُلْتُ لِعُرْوَةَ مَا بَالُ عَائِشَةَ تُتِمُّ فِي السَّفَرِ قَالَ إِنَّهَا تَأَوَّلَتْ كَمَا تَأَوَّلَ عُثْمَانُ ‏.‏
Reference : Sahih Muslim 685 c
In-book reference : Book 6, Hadith 3
USC-MSA web (English) reference : Book 4, Hadith 1460
  (deprecated numbering scheme)