Rashi, Deuteronomy 32:1

- Listen, O heavens - that I am warning Israel, and you shall be witnesses in this matter, for I have already told Israel that you will be witnesses. And so is [the clause] “And let the earth hear” [to be similarly understood]. Now why did [Moses] call upon heaven and earth to be witnesses [for warning Israel]? Moses said: “I am [just] flesh and blood. Tomorrow I will die. If Israel says, ‘We never accepted the covenant,’ who will come and refute them?” Therefore, he called upon heaven and earth as witnesses for Israel-witnesses that endure forever. Furthermore, if they [Israel] act meritoriously, the witnesses will come and reward them: “The vine will give its fruit,the earth will yield its produce, and the heavens will give their dew” (Zech. 8:12). And if [Israel] acts sinfully, the hand of the witnesses will be upon them first [to inflict punishment upon them]: “And He will close off the heaven that there will be no rain, and the soil will not give its produce” (Deut. 11:17), and then [the verse continues]: “and you will perish quickly”-through [the attacks of] the nations. — [Sifrei 32:1] [translation by Judaica Press]