Masturbation itself is never listed as a sin, however lustful thoughts are a sin.
Matthew 5:27–30
“You have heard that it was said, i‘You shall not commit adultery.’ 28 But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart."
The verses I would most associate with this type of sin, were I to be a Christian, would be in letters to the Corinthians and Romans:
“So whether you eat or drink or whatever you do, do it all for the glory of God” (1 Corinthians 10:31). It is difficult for me to see how jacking off would glorify this God.
“Everything that does not come from faith is sin” (Romans 14:23). I cannot see how jacking off would be something that comes "from faith".
“Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore, honor God with your body” (1 Corinthians 6:19–20). I think this one is obvious. How would you be honoring this God when you rubbed one out?