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