Dua by the traveller for the person wishing him farewell.

أَسْتَوْدِعِكُمُ اللَّهَ الَّذِي لَا تَضِيعُ وَدَائِعُهُ
"I leave your responsibility to Allah (leave you in his hands) in whose protection nothing can be lost (destroyed, damaged) when He protects".

(Hisnul Hasin)