(5) The Book of Salah
كتاب الصلاة
(3) Chapter: How The Salah Was Made Obligatory
(3) باب كَيْفَ فُرِضَتِ الصَّلاَةُ ‏.‏
Sunan an-Nasa'i 453 
It was narrated that 'Aishah said:
"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."
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، قَالَتْ أَوَّلَ مَا فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَأُتِمَّتْ صَلاَةُ الْحَضَرِ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 453
In-book reference : Book 5, Hadith 6
English translation : Vol. 1, Book 5, Hadith 454