Hadith 6

Sunan an-Nasa'i

Book 5: The Book of Salah Sahih

Chapter 3

How The Salah Was Made Obligatory

باب كَيْفَ فُرِضَتِ الصَّلاَةُ ‏.‏

Sahih
Sunan an-Nasa'i 453
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."