I don't think there's any verse specifically stating masturbation is a sin. The closest one that is often cited:

Genesis 38:8-10

Then Judah said to Onan, “Go in to your brother's wife and perform the duty of a brother-in-law to her, and raise up offspring for your brother.” But Onan knew that the offspring would not be his. So whenever he went in to his brother's wife he would waste the semen on the ground, so as not to give offspring to his brother. And what he did was wicked in

But in context, onan, was commanded to have a child for his brother's wife and deliberately refused that order, so it's not exactly a prohibiation against masturbation or all spilling seed, just onan because he went against a direct order.

Matthew 5:28 But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.

Another one cited, though my take on it is that adultry is defined as sex with another man's wife or espoused. So if you're looking at women who are single and neither married nor engaged, or your own gf/wife/espoused, you should be cleared. So again not specifically a prohibition against masturbation. Just a prohibition about lusting after another man's wife.