فَإِنَّ مَا يَمْضِي مِنْهُ لَا يَعُودُ.
For what passes of time does not return.
The initial fāʾ signals a reason supporting the preceding prohibition, rather than an unrelated new report. “Inna” reinforces the explanation. The phrase “mā yamḍī minhu” may be read with the relative mā, referring to the portion of time that passes; “lā yaʿūd” then predicates nonreturn of that portion.
Removing the rationale would leave the prohibition grammatically complete, but the audience would lose the explicit persuasive reason. The additional words therefore make a functional contribution and need not count as wasteful length.