[Machine] The prayer was prescribed as two units, but the Messenger of Allah ﷺ added in the prayer of noon and left the prayer of travel as it is.
فُرِضَتِ الصَّلَاةُ رَكْعَتَيْنِ فَزَادَ رَسُولُ اللهِ ﷺ فِي صَلَاةِ الْحَضَرِ وَتَرَكَ صَلَاةَ السَّفَرِ عَلَى نَحْوِهَا
The prayer was prescribed as two rak'ahs, two rak'ahs both in journey and at the place of residence. The prayer while travelling remained as it was (originally prescribed), but an addition was made in the prayer (observed) at the place of residence.
عَنْ عَائِشَةَ زَوْجِ النَّبِيِّ ﷺ أَنَّهَا قَالَتْ فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ رَكْعَتَيْنِ فِي الْحَضَرِ وَالسَّفَرِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَزِيدَ فِي صَلاَةِ الْحَضَرِ
"The first time the Salah was enjoined it was two Rak'ahs, and it remained as such when traveling, but the Salah while resident was made complete."
أَوَّلَ مَا فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَأُتِمَّتْ صَلاَةُ الْحَضَرِ
"The Salah was enjoined two Rak'ahs at a time, then the Salah when traveling remained like that, but the Salah while resident was increased."
فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ رَكْعَتَيْنِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَزِيدَ فِي صَلاَةِ الْحَضَرِ
The prayer was prescribed as consisting of two rak'ahs both when one was resident and when travelling. The prayer while travelling was left according to the original prescription and the prayer of one who was resident was enhanced.
فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ رَكْعَتَيْنِ فِي الْحَضَرِ وَالسَّفَرِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَزِيدَ فِي صَلاَةِ الْحَضَرِ
"The Salah was enjoined two Rak'ahs at a time, then the Salah when traveling remained like that, but the Salah while resident was increased." (Using translation from Nasāʾī 455)
«فُرِضَتِ الصَّلَاةُ رَكْعَتَيْنِ فَأُقِرَّتْ صَلَاةُ السَّفَرِ وَزِيدَ فِي صَلَاةِ الْحَضَرِ»