Is it obligatory to fulfill a vow of 300 rak'ahs as a punishment for masturbation?

If you utter the aforementioned covenant in the form of a vow, such as saying: "I covenant with Allah to abstain from this sin," then this falls under the category of a "vow of obstinacy" (نذر اللجاج). The majority of scholars hold that you are given the choice between fulfilling what you have committed to or offering an expiation for a broken oath (كفارة يمين). However, if the covenant was merely an internal thought without the formulation of a vow, then no expiation is required. What remains obligatory is to abandon the detestable sin that entails harm and risks.