When. At the farewell — said by the one staying behind to the one leaving. This is the rarer direction: most travel supplications are for the traveller to say himself. Here the words belong to the person on the platform.
The words. Astawdi'u is the verb of leaving something in trust, from wadi'ah, a deposit placed with someone for safekeeping and expected back. It is a commercial word before it is a devotional one. What is deposited is named in three parts, and the choice of the three is the whole supplication.
Dinaka — your religion, which travel loosens first. Amanataka — your trusts: what has been entrusted to you, your obligations, your family, the things you are answerable for. And khawatima 'amalik — the ends of your deeds, from khatm, a seal. Not the deeds themselves but how they finish.
What is not asked for. Nothing about safe roads, comfort, or a swift return. A person seeing off someone they love wants to say exactly those things, and the supplication does not say them. It goes past the journey to what the journey could cost.
Why the ends. The third item quietly widens the frame beyond this trip. Deeds are judged by how they conclude, and a farewell is the moment a person is most aware that any parting might be the last one.
Its counterpart. The traveller has his own supplication for mounting and setting out. This one is the answer from the other side, and the two are usually said within the same minute.