How the prayer was first given
One of the two was left as it was.
فَرَضَ اللَّهُ الصَّلاَةَ حِينَ فَرَضَهَا رَكْعَتَيْنِ رَكْعَتَيْنِ فِي الْحَضَرِ وَالسَّفَرِ، فَأُقِرَّتْ صَلاَةُ السَّفَرِ، وَزِيدَ فِي صَلاَةِ الْحَضَرِ
Faraḍa-Llāhu-ṣ-ṣalāta ḥīna faraḍahā rakʿatayni rakʿatayni fi-l-ḥaḍari wa-s-safar, fa-uqirrat ṣalātu-s-safar, wa zīda fī ṣalāti-l-ḥaḍar
When Allah made the prayer obligatory He made it two units and two units, at home and on a journey. Then the travelling prayer was left as it was, and the prayer at home was increased.
Where this comes from
Aisha reports it. The shortened prayer on a journey is not a reduction from the full one. It is the original, and the longer one is the addition.
The full narration
Narrated `Aisha:the mother of believers: Allah enjoined the prayer and 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
Source
Ṣaḥīḥ al-Bukhārī · 350
- Agreed upon as authentic (al-Bukhārī / Muslim)