Sahih al-Bukhari 350
Sahih al-Bukhari · Book 8: Prayers (Salat) · 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)
Narrated `Aisha:
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 " .
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.