What is the ruling on masturbation in Islamic law, and what are its health harms, given that the questioner has resolved to repent from it?

Masturbation is forbidden by Islamic law, according to the consensus of scholars. They cite as evidence the Quranic verses: "And those who guard their private parts, except from their wives or those whom their right hands possess, for indeed, they are not to be blamed. But whoever seeks beyond that – then those are the transgressors," and the saying of the Prophet, peace and blessings be upon him: "O young people, whoever among you can afford to marry, let him do so, for it is more effective in lowering the gaze and guarding the private parts. And whoever is unable to do so, then he should fast, for it will be a shield for him." Imam Ahmad's school of thought is particularly strict in prohibiting masturbation. Furthermore, masturbation has psychological and physical harms. One must repent from it immediately, as delaying repentance is from the whispers of Satan.