It was narrated that Ibn 'Abbas said:
"Allah, the Mighty and Sublime, enjoined the prayer on the tongue of your Prophet (ﷺ): While a resident four (rak'ahs), while traveling two, and at times of fear one."
Reference: Sunan an-Nasa'i 1442
In-book reference: Book 15, Hadith 10
English translation: Vol. 2, Book 15, Hadith 1443