'A'isha, the wife of the Messenger of Allah ﷺ, said Allah prescribed the prayer as two rak'ahs, then it was completed (to four rak'ahs) at the place of residence, but was retained in the same position in journey as it was first made obligatory.  

أَنَّ عَائِشَةَ زَوْجَ النَّبِيِّ ﷺ قَالَتْ فَرَضَ اللَّهُ الصَّلاَةَ حِينَ فَرَضَهَا رَكْعَتَيْنِ ثُمَّ أَتَمَّهَا فِي الْحَضَرِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ عَلَى الْفَرِيضَةِ الأُولَى  

"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  

"Urwah told me that 'Aishah said: 'Allah enjoined the salah upon the Messenger of Allah ﷺ, and the first thing that He enjoined was two Rak'ahs at a time, then it was made complete four Rak'ahs while in the state of residence but the prayer when traveling remained two Rak'ahs, as it was first enjoined.'"  

عَنْ صَلاَةِ رَسُولِ اللَّهِ ﷺ بِمَكَّةَ قَبْلَ الْهِجْرَةِ إِلَى الْمَدِينَةِ قَالَ أَخْبَرَنِي عُرْوَةُ عَنْ عَائِشَةَ قَالَتْ فَرَضَ اللَّهُ ﷻ الصَّلاَةَ عَلَى رَسُولِهِ ﷺ أَوَّلَ مَا فَرَضَهَا رَكْعَتَيْنِ رَكْعَتَيْنِ ثُمَّ أُتِمَّتْ فِي الْحَضَرِ أَرْبَعًا وَأُقِرَّتْ صَلاَةُ السَّفَرِ عَلَى الْفَرِيضَةِ الأُولَى