What is the general ruling on masturbation in Islam, and what is its ruling if it is with the wife's help due to special circumstances for him, such as sexual weakness, or if it prevents him from committing fornication?

Masturbation is impermissible due to its contravention of Islamic law and its detriment to health, unless one fears falling into Zina (unlawful sexual intercourse) and finds no other recourse but through it. In such a case, one commits the lesser of two harms. As for masturbating with one's wife's hand or with her body parts, it is permissible and falls under the general permissibility of marital enjoyment.