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كتاب الصلاة
8  
Prayers (Salat)
(1)
Chapter: How As-Salat (the Prayer) was prescribed on the night of Al-Isra (miraculous night journey) of the Prophet (pbuh) to Jerusalem (and then to the heavens)
(1)
باب كَيْفَ فُرِضَتِ الصَّلاَةُ فِي الإِسْرَاءِ
Sahih al-Bukhari 350

Narrated `Aisha:

the mother of believers: Allah enjoined the prayer when He enjoined it, it was two rak`at only (in every prayer) both when in residence or on journey. Then the prayers offered on journey remained the same, but (the rak`at of) the prayers for non-travelers were increased.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ أَخْبَرَنَا مَالِكٌ، عَنْ صَالِحِ بْنِ كَيْسَانَ، عَنْ عُرْوَةَ بْنِ الزُّبَيْرِ، عَنْ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ، قَالَتْ ‏ "‏ فَرَضَ اللَّهُ الصَّلاَةَ حِينَ فَرَضَهَا رَكْعَتَيْنِ رَكْعَتَيْنِ فِي الْحَضَرِ وَالسَّفَرِ، فَأُقِرَّتْ صَلاَةُ السَّفَرِ، وَزِيدَ فِي صَلاَةِ الْحَضَرِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 350
In-book reference : Book 8, Hadith 2
USC-MSA web (English) reference : Vol. 1, Book 8, Hadith 346
  (deprecated numbering scheme)