What is the expiation for one who vowed to Allah to abandon masturbation, then committed it six times, and how can he fulfill his vow when he is unable to do so?

Masturbation is forbidden due to its transgression and harm. One must avoid it and keep away from its causes, especially if a Muslim has made a covenant with his Lord to do so. Whoever has masturbated must repent to Allah, fast frequently, associate with righteous people, adhere to remembrance [dhikr] and supplication [du'a], and occupy themselves with what is beneficial. If the covenant to abstain was made verbally, it is an oath that requires a single expiation [kaffarah] upon breaking it. However, if it was merely a thought, then there is no expiation for it, but sincere repentance [tawbah nasuh] is obligatory.