Sunan an-Nasa'i #453 · The Book of Salah

Sunan an-Nasa'iIt was narrated that 'Aishah said
Sunan an-Nasa'i · Book 5 · The Book of Salah
Sunan an-Nasa'i · كتاب 5 · كتاب الصلاة
"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 ‏.‏
Reference : Sunan an-Nasa'i 453In-book reference : Book 5, Hadith 6