(1) باب
It was narrated that Ibn Abbas said:
"Allah (SWT) enjoined the prayer on the tongue of your Prophet (ﷺ): four (rak'ahs) while a resident, two rak'ahs while traveling, and one rak'ah during times of fear." (ID)
Reference (ID): Sunan an-Nasa'i 1532 (ID)
In-book reference (ID): Book 18, Hadith 4 (ID)
English translation (ID): Vol. 2, Book 18, Hadith 1533 (ID)