Tafsir of Ash-Shu`ara' 26:24

He [Musa], peace be upon him, said, turning away from answering him [directly] to mentioning His attributes, Exalted is He, in the manner of the uslub al-hakim (wise discourse), as an indication of the impossibility of explaining the reality [of God].

"The Lord of the heavens and the earth and whatever is between them."

The discussion regarding the impossibility of knowing [His] essence or otherwise has already passed you by, so remember it. The word Rabb (Lord) is in the nominative case (raf') as the predicate of an omitted subject; meaning: "He is the Lord of the heavens and the earth and whatever is between them of the elements and composite entities."

"If you are certain."

Meaning: If you are certain about things and are verifiers of them, you would know this. Or, if you are certain about anything at all, then this is more worthy of certainty due to its manifestness and the illumination of its proof. For these sensible celestial bodies are contingent (mumkina) due to their composition, multiplicity, and the changing of their states; therefore, they must have a Principle that is necessary by His Essence. Furthermore, that Principle must necessarily be the principle for all other contingent beings—both those that can be sensed and those that cannot—otherwise, it would necessitate a multiplicity of the Necessary [Being] or that some contingent beings are independent of Him, and both are impossible. The response to "if" (in) is omitted, as we have indicated.