Is expiation obligatory for one who swore by the Holy Qur’an not to return to masturbation, but then experienced an emission upon unintentionally seeing obscene images?

Masturbation is forbidden. If a person has sworn an oath not to practice it, he does not break his oath by ejaculating due to watching obscene scenes, and there is no expiation due upon him, even though what he did is forbidden. It is obligatory for a Muslim to lower his gaze from forbidden things, in compliance with the words of Allah Almighty: "Tell the believing men to lower their gaze and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do." It is also forbidden to copy obscene scenes or assist in doing so.