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." (DE)
Reference (DE): Sunan an-Nasa'i 1442 (DE)
In-book reference (DE): Book 15, Hadith 10 (DE)
English translation (DE): Vol. 2, Book 15, Hadith 1443 (DE)