بِسْمِ الله
Bismillāhi
In the Name of Allah.
At-Tirmidhi 2/505, among others. See 'Irwa'ul Ghalil no. 49 and Sahihul-Jami' 3/203
مَا يَقُولُ إِذَا وَضَعَ الثَّوْبَ
Before undressing
Related
When wearing a garmentWhen wearing a new garmentTo someone wearing a new garmentBefore entering the bathroomAfter leaving the bathroomRemembrance when leaving the homeRemembrance upon entering the homeWhen breaking fast in someone’s homeReturning a supplication of forgivenessFor travelUpon entering a town or villageWhen entering the marketSupplication of the traveller for the residentSupplication of the resident for the travellerStopping or lodging somewhere in travel and otherwiseWhile returning from travelReturning a greeting to a disbeliever