Sunan an-Nasa'i, Hadith 1532 ⓘ
It was narrated that Ibn Abbas said: "Allah (SWT) enjoined the prayer on the tongue of your Prophet: four (rak'ahs) while a resident, two rak'ahs while traveling, and one rak'ah during times of fear."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ بُكَيْرِ بْنِ الأَخْنَسِ، عَنْ مُجَاهِدٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ فَرَضَ اللَّهُ الصَّلاَةَ عَلَى لِسَانِ نَبِيِّكُمْ صلى الله عليه وسلم فِي الْحَضَرِ أَرْبَعًا وَفِي السَّفَرِ رَكْعَتَيْنِ وَفِي الْخَوْفِ رَكْعَةً .
also referenced under: Book 18, Hadith 4 (In-Book) & Volume 2, Book 18, Hadith 1533 (DarusSalam)