Rashi on Numbers 9:1/רש"י על במדבר ט׳:א׳

1בחדש הראשון [AND THE LORD SPAKE UNTO MOSES] IN THE FIRST MONTH — The section which appears at the commencement of this Book was not spoken before Eyar (the second month; cf. Numbers 1:1): you learn, therefore, that there is no “earlier” or “later” (no chronological order) in the Torah. But why, indeed, did not Scripture open the Book with this section? Because it implies something disparaging to Israel (Sifrei Bamidbar 64:1) — that during all the forty years they were in the wilderness they offered only this single Passover sacrifice (Tosafot on Kiddushin 38b s. v. הואיל)