سُنَنُ النَّسَائِيِّ
Sunan an-Nasa’i 453
The Book of Salah
How The Salah Was Made Obligatory
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 .
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."
Sunan an-Nasa'i, 453Sahih (Darussalam)