(3) باب كَيْفَ فُرِضَتِ الصَّلاَةُ .
It was narrated that 'Aishah said:
"The Salah was enjoined two Rak'ahs at a time, then the Salah when traveling remained like that, but the Salah while resident was increased."
Reference: Sunan an-Nasa'i 455
In-book reference: Book 5, Hadith 8
English translation: Vol. 1, Book 5, Hadith 456