Ibn ‘Abbas reported:
Allah has prescribed the prayer through the word of your Prophet (ﷺ) as four rak’ahs when resident, two when travelling, and one when danger is present.
Reference : Sahih Muslim 687 a
In-book reference : Book 6, Hadith 6
USC-MSA web (English) reference : Book 4, Hadith 1463
(deprecated numbering scheme)