There are no explicit declarations that I know of in the Old Testament that the Messiah would be a prophet, but it is implied. Looking at Isaiah 11:2-4,
2 The Spirit of the LORD will rest on him—
the Spirit of wisdom and of understanding,
the Spirit of counsel and of power,
the Spirit of knowledge and of the fear of the LORD -
3 and he will delight in the fear of the LORD.
He will not judge by what he sees with his eyes,
or decide by what he hears with his ears;
4 but with righteousness he will judge the needy,
with justice he will give decisions for the poor of the earth.
He will strike the earth with the rod of his mouth;
with the breath of his lips he will slay the wicked.
The Messiah will have extraordinary ability to judge and will have deep knowledge of the Lord, two qualities that characterized prophets in the Old Testament.
In addition, there is Deut 18:15
"The LORD your God will raise up for you a prophet like me from among your own brothers. You must listen to him."
There are no figures in the Old Testament that have a standing as high as Moses, but here he states that one will come who must be heeded as he was. So one might wonder if there are two authoritative figures to come, or both are embodied in the same person. The characteristics of the Messiah would tend to favor the one person.
