(4) Prayer
كتاب الصلاة
(42a) Chapter: Prayer while Travelling - Section 1
(41) باب صلاة السفر - الفصل الأول
Mishkat al-Masabih 1340 
Ibn ‘Umar said that on a journey God’s Messenger would pray on his riding beast in whatever direction it turned, making a sign with his head* in prayer during the night, but not in the obligatory prayers; and he would observe a witr on his riding beast. *To indicate the bowing and the prostration. (Bukhari and Muslim.)
وَعَنِ ابْنِ عُمَرَ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّي فِي السَّفَرِ عَلَى رَاحِلَتِهِ حَيْثُ تَوَجَّهَتْ بِهِ يُومِئُ إِيمَاءً صَلَاةَ اللَّيْلِ إِلَّا الْفَرَائِضَ وَيُوتِرُ على رَاحِلَته
 مُتَّفَقٌ عَلَيْهِ  (الألباني)حكم :
Reference : Mishkat al-Masabih 1340
In-book reference : Book 4, Hadith 747