(3) باب كَيْفَ فُرِضَتِ الصَّلاَةُ .
Abu 'Amr - meaning, Al-Awza'i - said that he asked Az-Zuhri about the prayer of the Messenger of Allah (ﷺ) in Makkah before the Hijrah to Al-Madinah. He said:
"Urwah told me that 'Aishah said: 'Allah enjoined the salah upon the Messenger of Allah (ﷺ), and the first thing that He enjoined was two Rak'ahs at a time, then it was made complete four Rak'ahs while in the state of residence but the prayer when traveling remained two Rak'ahs, as it was first enjoined.'"
Reference: Sunan an-Nasa'i 454
In-book reference: Book 5, Hadith 7
English translation: Vol. 1, Book 5, Hadith 455