Leviticus 20:13 - If a man also lie with mankind, as he lieth with a woman, both of them have committed an abomination: they shall surely be put to death; their blood shall be upon them.
Remember, though, that Leviticus ostensibly itemizes laws that applied exclusively to the Jews. Instead, look to the books of Corinthians and Timothy for New Testament exclusions of those who practice homosexuality.
Romans 1:
18The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, 19since what may be known about God is plain to them, because God has made it plain to them. 20For since the creation of the world God's invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that men are without excuse.
21For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened. 22Although they claimed to be wise, they became fools 23and exchanged the glory of the immortal God for images made to look like mortal man and birds and animals and reptiles.
24Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. 25They exchanged the truth of God for a lie, and worshiped and served created things rather than the Creator—who is forever praised. Amen.
26Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones. 27In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion.
28Furthermore, since they did not think it worthwhile to retain the knowledge of God, he gave them over to a depraved mind, to do what ought not to be done. 29They have become filled with every kind of wickedness, evil, greed and depravity. They are full of envy, murder, strife, deceit and malice. They are gossips, 30slanderers, God-haters, insolent, arrogant and boastful; they invent ways of doing evil; they disobey their parents; 31they are senseless, faithless, heartless, ruthless. 32Although they know God's righteous decree that those who do such things deserve death, they not only continue to do these very things but also approve of those who practice them.
No need to cite just the Old Testament.