فَإِذَا وُضِعَ فِي لَحْدِهِ قَالَ الَّذِي يَضَعُهُ: «بِاسْمِ اللَّهِ وَعَلَى مِلَّةِ رَسُولِ اللَّهِ»
Translation: When the deceased is placed in the lahd, the person lowering the body says: In the name of Allah, and upon the religion of the Messenger of Allah.
Bismillah invokes the name of Allah; ‘ala millat Rasul Allah refers to the faith brought by the Messenger ﷺ.
The phrase belongs to the act of lowering, not to the funeral prayer takbirs, nor is it a condition for the burial’s validity.