فَلَآ أُقْسِمُ بِرَبِّ ٱلْمَشَـٰرِقِ وَٱلْمَغَـٰرِبِ إِنَّا لَقَـٰدِرُونَ
Falaaa uqsimu bi Rabbil mashaariqi wal maghaaribi innaa laqaadiroon
and, by the Lord of every sunrise and sunset, We have the power
So I swear by the Lord of the easts and the wests — indeed, we have the power
The oath is sworn on plurals: easts and wests, a rising point and a setting point for every day of the year, which makes the sun's yearly travel along the horizon the witness. What it swears to is the power of replacement, spoken to people who had just been asked whether each of them expects the garden. Grammarians divide over the particle in la uqsimu, whether it negates, rebuts what came before, or only strengthens what follows.