Dua of the traveler for those left behind

When. At the parting itself — said by the one leaving, to the family, the household, the people staying behind. It is short enough to say at a door or a departure gate, which is where partings actually happen.

The words. Astawdi'u is the tenth form of the root w-d-'a, the root of wadi'ah: a deposit, something handed to a trustee to hold and return intact. The verb is what you do when you leave valuables with someone before a journey. The traveller uses it about people, and about Allah as the One receiving them. Then the relative clause does the reassuring: alladhi la tadi'u wada'i'uh — 'whose deposits are never lost'. Anyone who has left something with a friend knows the fear the clause is answering.

Why the leaver says it. The person departing is the one who can do least. He is removing himself from the house, from the illness that might come, from the day his children have without him. The supplication converts an absence into a transfer of custody, which is the only thing in his power at that moment.

The reply from the other side. Those staying behind have their own words for the traveller — entrusting his religion, his trusts and the ends of his deeds to Allah. Said both ways, the parting has each side holding the other in the same place, and neither is left merely waving.

A note on scope. It is not restricted to long journeys or to danger. A commute, a hospital stay, a child moving away — the sentence fits wherever someone is being left.