Here are a couple:
Tertullian said (in Against Marcion): "But we do confess that a kingdom is promised to us upon the earth, although before heaven, only in another state of existence; inasmuch as it will be after the resurrection for a thousand years in the divinely-built city of Jerusalem…Of the heavenly kingdom this is the process: After its thousand years are over, within which period is completed the resurrection of the saints, who rise sooner or later according to their deserts, there will ensue the destruction of the world and the conflagration of all things at the judgment."
Justin Martyr said (in
Dialogue with Trypho ): “But I and others, who are right-minded Christians on all points, are assured that there will be a resurrection of the dead, and a thousand years in Jerusalem, which will then be built, adorned, and enlarged, as the prophets Ezekiel and Isaiah and others declare.”
http://cicministry.org/scholarly/sch008.htm <- here is a good link with quotes and sources. I like specifically how Irenaeus predicted spiritualization of prophecy: "“If, however, any shall endeavor to allegorize prophecies of this kind, they shall not be found consistent with themselves in all points.”
“If, then, God promised him the inheritance of the land, yet he did not receive it during all the time of his sojourn there, it must be, that together with his seed, that is, those who fear God and believe in Him, he shall receive it at the resurrection of the just.”
But one of the earliest is the Didache, which im sure you know of, its a very early document and it hints at a millennium because it says " And then shall appear the signs of the truth; first the sign of the appearance in heaven, then the sign of the sound of the trumpet, and thirdly the resurrection of the dead
not of all, but as it has been said, The Lord shall come and all his saints with him;" (this would line up with the first resurrection of Rev20)