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)
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 ".
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.