What is the sin of masturbation and what is its expiation for someone who is unmarried and unable to stop it?

The ruling on masturbation was previously discussed in a fatwa. One should not think too highly of oneself and attribute it to righteousness, for Allah the Almighty says: "So do not assert purity for yourselves. He is most knowing of who is righteous" [An-Najm:32]. There is no harm in feeling joy for an act of obedience and sadness for a sin. There is no expiation for engaging in masturbation; rather, what is obligatory is repentance with its three conditions: remorse for what has passed, a firm resolve not to return to the sin, and immediately abandoning it. The believer must hasten to repent and seek Allah's help. If he returns to the sin, he must repent again.