"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."
Sunan an-Nasa'i Book 5, Hadith 6
← Previous Hadith It was narrated from Anas bin Malik that the prayers were enjoined in Makkah, and that two angels came to the Messenger of Allah (ﷺ) and took him to Zamzam, where they split open his stomach and took out his innards in a basin of gold, and washed them with Zamzam water, then they filled his heart with wisdom and knowledge.

Sunan an-Nasa'i Book 5, Hadith 5 Next Hadith → "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.'"

Sunan an-Nasa'i Book 5, Hadith 7