Midrasch zu Michah 1:2

Variantly: "Hear, O heavens, etc.": Moses appointed over Israel two witnesses (heaven and earth) that live forever, telling them: I am flesh and blood; tomorrow I will die. If they wish to say that they never received the Torah, who will come and refute them? Therefore, He appointed for them two witnesses that live forever. And the Holy One Blessed be He made this (Ha'azinu) song a witness, viz. (Devarim 31:19) "so that this song be for Me a witness in the children of Israel." He said: The sun will be witness for them below, and I, above. And whence is it derived that G-d is called "a swift witness"? From (Malachi 3:5) "And I will draw near to you in judgment, and I will be a swift witness," and (Jeremiah 29:23) "and I am the one who knows and bears witness," and (Michah 1:2) "and the L-rd G-d will be a witness against you."