Revelation 20:4, A thousand years. This is but one of six references in the Bible to the Millennium lasting for one thousand years. The other references are found in the surrounding verses (vv. 2, 3, 5, 6, 7). These are more than enough references to meet the biblical criteria of establishing a truth in the mouth of two or more witnesses.
Even then, some biblical students still question the idea of the Millennium and categorize it as a NT concept only and, therefore, feel they can disregard it. This is a shame and shows naive ignorance with regard to the Word of Elohim.
An understanding of the prophetic implications of the seventh day Sabbath should settle this discussion once for all. In Hebraic thought, the seven days of creation picture the time man will be on this physical earth. Six days (for six days or six thousand years) man does his own work, while on the seventh day (or one thousand years) he rests and fellowships with Elohim. This is a picture of the millennial rest, which the Jewish sages have long called the Messianic Era, which is to come.
The Millennium is not uniquely a Christian or NT concept, but is rooted in Judaism going back before the Christian era. The writer of Hebrews makes reference to this in Heb 7 when he compares Joshua taking the Israelites into the Promised Land with Yeshua taking the saints into the Sabbath, millennial rest of our spiritual inheritance.