Sunan An-Nasa'i - سنن النسائي
5. The Book of Salah - كتاب الصلاة
3) Chapter: How The Salah Was Made Obligatory
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."
3) باب كَيْفَ فُرِضَتِ الصَّلاَةُ .
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 .
