Mishkat al-Masabih 4 Prayer كتاب الصلاة 757

وَعَن ابْن عَبَّاس وَعَنِ ابْنِ عُمَرَ قَالَا: سَنَّ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ صَلَاةَ السَّفَرِ رَكْعَتَيْنِ وَهُمَا تَمَامٌ غَيْرُ قَصْرٍ وَالْوِتْرُ فِي السَّفَرِ سنة. رَوَاهُ ابْن مَاجَه

He and Ibn ‘Umar said that God’s Messenger established the practice of praying two rak’as when travelling, they constituting a complete observance and not an abbreviation; and the witr while travelling is a sunna . Ibn Majah transmitted it.