Within the prayer window, the precise cause of performance becoming binding is the portion of time immediately connected to starting the prayer. The technical phrase is Juz' Muttasil bil-Tahrimah1.
This explains the two death examples:
• If a person dies before the prayer time begins, that prayer has not become obligatory on them.
• If the person dies within the prayer window, before performing it, they are not held to account for omitting that prayer in the case described. The explanation connects the cause to the portion immediately adjoining commencement, rather than treating the obligation as already fixed before that portion.
1. Juz' Muttasil bil-Tahrimah: the portion of time immediately adjoining the opening of prayer; Tahrimah refers here to entering prayer with its opening Takbir.
- Which Part of Prayer Time Is the Cause?