(1) باب صَلاَةِ الْمُسَافِرِينَ وَقَصْرِهَا
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. (ID)
Reference (ID): Sahih Muslim 687 a (ID)
In-book reference (ID): Book 6, Hadith 6 (ID)
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1463 (ID)