Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that on a journey he would not pray anything with the obligatory prayers, neither before them nor after them; he would only pray the night prayer (tahajjud), and that too on his mount, whichever direction it faced.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5508