Where in the Holy Bible is it stated that a believer should not get married to an unbeliever?
From the time of Moses, God not only forbade believers to marry unbelievers, but asked His people to destroy all the inhabitants of the land where they were going to lest they marry from among them; "and you take of his daughters for your sons, and his daughters play the harlot with their gods and make your sons play the harlot with their gods" (Ex 34:16).
In the time of Joshua we read; "Therefore take careful heed to yourselves, that you love the Lord your God. Or else, if indeed you do go back, and cling to the remnant of these nations; these that remain among you; and make marriages with them, and go in to them and they to you, know for certain that the Lord your God will no longer drive out these nations from before you" (Josh 23:11-13).
In the Holy Book of Ezra two whole chapters 9 and 10 deal with the issue of marrying pagans. Ezra torn his garment and fasted and prayed when he heard that the people of Israel took for themselves wives from the peoples of the lands. Nehemiah made the people go into covenant saying "We would not give our daughters as wives to the peoples of the land, nor take their daughters for our sons" (Neh 10:30). Those are few examples of forbidding intermarriage between believers and non believers.
In the New Testament, St. Paul teaches us "Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness? And what accord has Christ with Belial? Or what part has a believer with an unbeliever? And what agreement has the temple of God with idols? For you are the temple of the living God. As God has said: "I will dwell in them And walk among them. I will be their God, and they shall be My people." Therefore "Come out from among them and be separate, says the Lord. Do not touch what is unclean, and I will receive you. I will be a Father to you, and you shall be My sons and daughters, Says the Lord Almighty" (2 Cor 6:14-18).