It's the passage that is most often translates to masturbation=sin. Though it doesn't refer specifically to fapping it universally applies to any action, activity, event that gives you pleasure. Anything that gives you selfsatisfaction (socializing, playing vidya, watching tv, banging your wife/waifu bodypillow, attending furry conventions, THE WORKS) is sinful.
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Not a citation, that interpretation and relation of the passage but Kay