بُلُوغُ الْمَرَامِ
Bulugh al-Maram 483
The Book of Prayer
وَعَنِ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ صَلَاةُ اَلْخَوْفِ رَكْعَةٌ عَلَى أَيِّ وَجْهٍ كَانَ } رَوَاهُ اَلْبَزَّارُ بِإِسْنَادٍ ضَعِيفٍ [1] .
Narrated Ibn 'Umar (RA): Allah's Messenger (ﷺ) said: "The prayer in time of danger is one Rak'ah, in whatever manner it is (performed)." [al-Bazzar reported it through a weak chain of narrators].
Sunnah.com reference: Book 2, Hadith 408