Rashi

Hear, O heavens, and give ear, O earth And Moses said, “Give ear, O heavens,...and may the earth hear” (Deut. 32:1). Why did Isaiah change the wording? Our masters taught concerning this matter, [and] many midrashim [are] in the section entitled “Ha’azinu” in Sifrei, but the Sages disagreed with them and said: A matter is not so unless witnesses come and testify. If their words coincide, their testimony is fulfilled; if not, their testimony is not fulfilled. Had Isaiah not addressed the heavens with giving ear and the earth with hearing, the heavens would testify and say, ‘When we were called to this testimony in Moses’ time, when he said, (Deut. 30:19) “I call heaven and earth to witness against you,” we heard with an expression of giving ear,’ and the earth would testify, ‘I was called with an expression of hearing,’ hence their testimony would not coincide. [Therefore,] Isaiah came and reversed the matter. Consequently, both are found to testify with an expression of giving ear and with an expression of hearing. for the Lord has spoken That you should be witnesses in this matter, when I warned them in Moses’ time. Therefore, come and hear what I reason with them, for they transgressed the warning, I did not sin against them, but I raised them and exalted them, yet they rebelled against me. [Another version reads:] That you should be witnesses in this matter. Now, where did He speak? “Give ear, O heavens and I will speak” (ibid. 32:1). [So this was taught] in Mechilta (Bo 12).