Tafsir of Al-Insan 76:23

When the Exalted—glorified be His Majesty—first mentioned the state of man, dividing him into the obedient and the disobedient, and elaborated on what He has prepared for the obedient, pointing to the immense breadth of His mercy, He then mentioned that with which He honored His Prophet to remove his alienation and strengthen his heart. He said, with Might as the Speaker: "Indeed, It is We who have sent down the Qur’an to you in stages."

That is, We sent it down divided and portioned over approximately twenty-three years, for profound wisdom requiring it, and not otherwise. This is indicated by the repetition of the pronoun, whether the detached pronoun is for emphasis, as a separator, or as an initial subject.