if one is traveling a distance over which it is permissible to shorten Salah, one should limit oneself to offering obligatory Salah and leave off supererogatory Prayers. It is authentically reported from
Ibn `Umar (may Allah be pleased with them both) that he said,
"I accompanied the Prophet (peace be upon him) in travel and did not see him offering Subhah (supererogatory Salah) and Allah Glory be to Him, says:
Indeed in the Messenger of Allâh (Muhammad صلى الله عليه وسلم) you have a good example to follow
The meaning of Subhah is Nafilah (supererogatory) Prayer. As for the two Rak`ah (unit of Prayer) before Fajr (Dawn) Prayer and Witr (Prayer with an odd number of units), one should offer them in travel or residence, according to the Hadith that is authentically reported from
`Aishah (may Allah be pleased with her) who said,
"The Prophet (peace be upon him) offered the ‘Isha’ (Night) prayer (and then got up at the Tahajjud time) and offered eight Rak‘ahs and then offered two Rak‘ahs while sitting. He then offered two Rak‘ahs in between the Adhan and Iqamah (of the Fajr prayer) and he never missed them."
Also it is authentically reported on the authority of
`Abdullah ibn `Amer
that his father told him that "the Prophet (peace be upon him) prayed Subhah at night on the back of his mount on a journey, facing whatever direction it took."
(Part No. 6; Page No. 115) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.