Seeing Someone Off
أَسْتَوْدِعُ اللَّهَ دِينَكَ وَأَمَانَتَكَ وَخَوَاتِيمَ عَمَلِكَ
Astawdi'ullaha dinaka wa amanataka wa khawatima 'amalik.
I place in Allah's keeping your religion, your trusts and the final outcomes of your deeds.
What it means
Notice what is entrusted to Allah: not the luggage or the flight, but the traveller's religion and the end of his deeds. Distance from home is where people change, so the farewell asks for the part of him that matters most to come back intact.
When and how
Say it to anyone leaving on a journey, at the door or at the airport.
Its virtue
The Prophet ﷺ would say this when bidding farewell to someone setting out, reported in Sunan Abi Dawud 2600 and Jami at-Tirmidhi 3443, where he said that Allah does not lose what is entrusted to Him.
Source
Abu Dawud 2600; At-Tirmidhi 3443sahih