Language:
كتاب التقصير
18
Shortening the Prayers (At-Taqseer)
(5)
Chapter: When a traveller leaves his original place, he can shorten his Salat (prayer)
(5)
باب يَقْصُرُ إِذَا خَرَجَ مِنْ مَوْضِعِهِ
Sahih al-Bukhari 1090

Narrated `Aisha:

"When the prayers were first enjoined they were of two rak`at each. Later the prayer in a journey was kept as it was but the prayers for non-travelers were completed." Az-Zuhri said, "I asked `Urwa what made Aisha pray the full prayers (in journey)." He replied, "She did the same as `Uthman did."

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتِ الصَّلاَةُ أَوَّلُ مَا فُرِضَتْ رَكْعَتَيْنِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ، وَأُتِمَّتْ صَلاَةُ الْحَضَرِ‏.‏ قَالَ الزُّهْرِيُّ فَقُلْتُ لِعُرْوَةَ مَا بَالُ عَائِشَةَ تُتِمُّ قَالَ تَأَوَّلَتْ مَا تَأَوَّلَ عُثْمَانُ‏.‏
Reference : Sahih al-Bukhari 1090
In-book reference : Book 18, Hadith 11
USC-MSA web (English) reference : Vol. 2, Book 20, Hadith 196
  (deprecated numbering scheme)