Salim narrates from his father (Sayyiduna Abdullah bin Umar) that I saw the Prophet Muhammad (peace be upon him) performing voluntary prayers on his mount during travel, and he was indicating (bowing and prostrating) with the movement of his head.
Hadith Referenceصحیح ابن حبان / كتاب الصلاة / 2522
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - انظر ما بعده.فضيلة الشيخ العلّامة شُعيب الأرناؤوطرجاله ثقات رجال الصحيح إلا أن فيه عنعنة الوليد بن مسلم