"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  

"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 Salah was enjoined on the lips of the Prophet ﷺ, four Rak'ahs while resident, and two while traveling, and one Rak'ah during times of fear."  

فُرِضَتِ الصَّلاَةُ عَلَى لِسَانِ النَّبِيِّ ﷺ فِي الْحَضَرِ أَرْبَعًا وَفِي السَّفَرِ رَكْعَتَيْنِ وَفِي الْخَوْفِ رَكْعَةً  

"How can the Salah be shortened as Allah says: There is no sin on you if you shorten As-Salah (the prayer) if you are in fear?" [1] Ibn 'Umar said: "O son of my brother! The Messenger of Allah ﷺ came to us when we had gone astray and he taught us. One of the things that he taught us was that Allah, the Mighty and Sublime, has commanded us to pray two Rak'ahs when traveling." [1] An-Nisa' 4:101.  

أَنَّهُ قَالَ لاِبْنِ عُمَرَ كَيْفَ تَقْصُرُ الصَّلاَةَ وَإِنَّمَا قَالَ اللَّهُ ﷻ { فَلَيْسَ عَلَيْكُمْ جُنَاحٌ أَنْ تَقْصُرُوا مِنَ الصَّلاَةِ إِنْ خِفْتُمْ } فَقَالَ ابْنُ عُمَرَ يَا ابْنَ أَخِي إِنَّ رَسُولَ اللَّهِ ﷺ أَتَانَا وَنَحْنُ ضُلاَّلٌ فَعَلَّمَنَا فَكَانَ فِيمَا عَلَّمَنَا أَنَّ اللَّهَ ﷻ أَمَرَنَا أَنْ نُصَلِّيَ رَكْعَتَيْنِ فِي السَّفَرِ قَالَ الشُّعَيْثِيُّ وَكَانَ الزُّهْرِيُّ يُحَدِّثُ بِهَذَا الْحَدِيثِ عَنْ عَبْدِ اللَّهِ بْنِ أَبِي بَكْرٍ