(3) باب كَيْفَ فُرِضَتِ الصَّلاَةُ .
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."
Reference: Sunan an-Nasa'i 453
In-book reference: Book 5, Hadith 6
English translation: Vol. 1, Book 5, Hadith 454