It is reported that al-Hasan said: “The Companions of the Messenger of Allah, peace and blessings be upon him, would travel (1) and offer voluntary prayers before and after the obligatory prayer (2).” This is also reported from ‘Umar (3), ‘Alī, Ibn Mas‘ūd, Jābir, Anas, Ibn ‘Abbās and Abū Dharr (4).
As for Ibn ‘Umar, he would not offer voluntary prayers before or after the obligatory prayer, except in the depths of the night (5), along with Witr. This appears to have been the Prophet’s practice, peace and blessings be upon him: he would not offer any prayer before or after the shortened obligatory prayer, but he did not prohibit voluntary prayer before or after it. Such prayer therefore counts as unrestricted voluntary prayer, not as a regular sunnah prayer attached to the obligatory prayer, like those offered when not travelling.
This is supported by the fact that the four-rak‘ah prayer has been reduced to two rak‘ahs to ease the burden on the traveller. How, then, could it have a regular sunnah prayer to be consistently observed, when the obligatory prayer itself has been reduced to two rak‘ahs? Had easing the burden on the traveller not been intended (6), it would have been more appropriate for him to pray in full. Hence the statement of