Abdur Rahman bin Mahdi عبد الرحمن بن مهدي · The Book of Salah

"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.'"