What is the oldest prophecy uttered by a prophet?
The prophet in question is Enoch. You find his prophecy not in Genesis but in Jude 1:14-16. The subject matter? The 2nd coming of Christ.
Jude 1:14-16 And Enoch also, the seventh from Adam, prophesied of these, saying, Behold, the Lord cometh with ten thousands of his saints, To execute judgment upon all, and to convince all that are ungodly among them of all their ungodly deeds which they have ungodly committed, and of all their hard speeches which ungodly sinners have spoken against him. These are murmurers, complainers, walking after their own lusts; and their mouth speaketh great swelling words, having men’s persons in admiration because of advantage.