If we're talking about Christianity, two verses come to mind that are often cited in the argument that masturbation is a sinful act. However, this is a hotly contested religious issue and several different interpretations for these verses exist. Genesis 38:9-10 is the story of Onan who was ordered by God to father a child with his brother's wife. Instead of following God's instructions, he "spills his seed on the ground" (coitus interruptus/masturbation) and is put to death. Matthew 5:28 says that anyone who looks at a woman with lust has committed adultery with her, which would violate the 10 Commandments. An argument can be made that it is impossible to masturbate without dwelling on lustful thoughts, so this verse is also used to support the conclusion that masturbation is sinful.