One should recite the following Duaa when bidding someone farewell at the time he departs on a journey
زَوَّدَكَ اللَّهُ التَّقْوَى وَغَفَرَ ذَنْبَكَ وَيَسَّرَ لَكَ الخَيْرَ حَيْثُمَا كُنْتَ
May Allah Ta‘ala grant you the provision of taqwa (during your journey), and forgive your sins, and may He make carrying out good deeds easy for you wherever you are
عن أنس، قال: جاء رجل إلى النبي صلى الله عليه وسلم فقال: يا رسول الله، إني أريد سفرا فزودني. قال: زودك الله التقوى، قال: زدني، قال: وغفر ذنبك قال: زدني بأبي أنت وأمي، قال: ويسر لك الخير حيثما كنت (ترمذي رقم 3444)
Hazrat Anas (radhiyallahu ‘anhu) reports that once a person came to Rasulullah (sallallahu ‘alaihi wasallam) and said, “O Rasulullah (sallallahu ‘alaihi wasallam), I intend embarking on a journey, thus make a duaa for me which will serve as my provisions during the journey.” Rasulullah (sallallahu ‘alaihi wasallam) replied, “May Allah Ta‘ala grant you the provision of taqwa.” The person said, “Grant me more (duaa).” Rasulullah (sallallahu ‘alaihi wasallam) said, “And forgive your sins.” The person said, “Grant me more (duaa), may my father and mother be sacrificed for you.” Rasulullah (sallallahu ‘alaihi wasallam) replied, “And may He make carrying out good deeds easy for you wherever you are.”
Source: Ihyaauddeen.co.za