Does masturbating out of forgetfulness invalidate the fast?

Masturbation (self-gratification) is forbidden and invalidates the fast. Its expiation is sincere repentance, involving cessation, regret, and a firm resolve not to return to it.

If a person stops masturbating before ejaculation, their fast is valid. However, if they ejaculate, and if they did so forgetfully, their fast is valid according to many scholars, based on the Prophet Muhammad's (peace be upon him) saying: "Forgiven for my Ummah are mistakes, forgetfulness, and what they are compelled to do."

For the fast to be invalidated intentionally, three conditions are required: knowledge, remembrance (non-forgetfulness), and volition.