Is it permissible for a wife to massage her husband's prostate gland by inserting her finger into his anus, knowing that it is forbidden for a woman to engage in anal intercourse?

Allah has forbidden inserting a finger into the anus because it is a means to sodomy. Some scholars have explicitly stated its prohibition. Among the evidences for its prohibition are:

Ibn al-Haj said: "One must beware of inserting his finger into his anus, for it is among the actions of the worst of people. It is forbidden because he does it to himself, and that is unlawful." Hashiyat al-Dusuqi: "There is no ghusl required for what is internal to the exit point, rather it is forbidden due to its resemblance to sodomy." It involves contact with severe impurity without necessity. It is considered a breach of honor (khawārim al-murū'ah) and leads to the degradation of a man's prestige in the eyes of his wife.

As for the alleged benefits of this act, there is no evidence for them. Rather, it has severe health harms, including: 1. Contracting hemorrhoids. 2. Suffering from tears and fissures inside the anus. 3. The possibility of contracting infections.