Praise be to Allah.
It is permissible for a man to enjoy his wife in whatever way he wants, so long as he avoids having intercourse at the time of her period and anal intercourse, because of the report narrated by Muslim (302) from Anas: that that when a woman among them menstruated, the Jews would not eat with her or sit with her in their houses. So the companions of the Prophet (peace and blessings of Allaah be upon him) asked the Prophet (peace and blessings of Allaah be upon him) about that, and Allaah revealed the words (interpretation of the meaning):
“They ask you concerning menstruation. Say: that is an Adha (a harmful thing for a husband to have a sexual intercourse with his wife while she is having her menses), therefore, keep away from women during menses”
The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Do everything except intercourse.” News of that reached the Jews and they said: “This man does not want to leave any of our affairs without differing from it.”
Al-Tirmidhi (135), Abu Dawood (3904) and Ibn Maajah (639) narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever has intercourse with a menstruating woman or with a woman in her back passage, or comes to a soothsayer, has disbelieved in that which was revealed to Muhammad (peace and blessings of Allaah be upon him).” Classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
Ahmad (9731) and Abu Dawood (2162) narrated that Abu Hurayrah said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Cursed be the one who has intercourse with his wife in her back passage.” This hadeeth was classed as hasan by Shu’ayb al-Arna’oot in Tahqeeq al-Musnad and by al-Albaani in Saheeh Abi Dawood.
With regard to putting one’s finger in the anus, the least that can be said is that it is makrooh, because it involves direct contact with impure matter (najaasah) and filth, and because it may lead one to do that which is obviously haraam, namely intercourse in the back passage. The Prophet (peace and blessings of Allaah be upon him) said:
“Whoever is cautious in the dubious matters will save his religious commitment and his honour, but whoever falls into dubious matters will fall into haraam, like a shepherd who grazes his flocks around a private pasture and is liable to enter it at any moment.” Narrated by al-Bukhaari, 52; Muslim, 1599.
The above view that it is makrooh is supported by the fact that the scholars explained the reason why anal intercourse is forbidden is the presence of filth. Ibn Qudaamah said: Because the Sunnah refers to the prohibition on anal intercourse, therefore it is forbidden. And it is forbidden because of the impurity, and that applies only to intercourse in the back passage.
This undoubtedly also applies to inserting the finger into the anus.
What Allaah has permitted concerning which there is no doubt is sufficient.
See also question no. 40520.
And Allaah knows best.