One should not insert finger into their partner’s back passage because inserting the finger into the anus will make it come into unnecessary direct contact with unclean fecal matter, it is something that is abhorrent to the sound fitrah (innate nature of man), and persisting in doing that may lead a person to do that which is worse than that, namely anal intercourse.

Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author states that: “It is permissible for each spouse to enjoy the entire body of the other, looking and touching, even the private part. Allah Almighty says: “They are raiment for you and you are raiment for them.” (al-Baqarah 2:187)

With regard to the husband engaging in foreplay with his wife in this manner, this may mean one of two things:

Either touching the anal sphincter, or by inserting his finger into the anus. 

With regard to simply touching the anal sphincter, there is nothing wrong with that, but it is better to avoid doing that because that may lead to something else.
With regard to inserting the finger into the anus, this should not be done, for several reasons:
1. The anus is the site of uncleanness and feces.
2. One of the reasons why anal intercourse is forbidden is so that the penis will not come into contact with unclean fecal matter; by the same token inserting the finger into the anus will make it come into unnecessary direct contact with unclean fecal matter.

3. This action is something that is abhorrent to the sound fitrah (innate nature of man) and offends good taste. Rather, it is blind imitation of those whose fitrah is corrupt and whose tastes are nasty, who do everything that they think of to satisfy their animalistic desires with no care for good manners, morals or cleanliness, and their desires make them see something foul as something good.

4. Persisting in doing that may lead a person to do that which is worse than that, namely anal intercourse. This is the habit of those who follow their desires and do whatever appeals to them, so they gradually fall into more serious things because of the less serious things that they are attracted to. Thus, they move step by step until they end up committing “lesser sodomy” (intercourse with a woman in her back passage). The Prophet (peace and blessings of Allah be upon him) described that by means of a brilliant analogy when he said: “What is permissible is clear and what is forbidden is clear, and between them are dubious matters of which many people are not aware. Whoever is cautious in the dubious matters will save his religious commitment and his honor, but whoever falls into dubious matters will fall into haram, like a shepherd who grazes his flocks around a private pasture and is liable to enter it at any moment. Every king has a sanctuary, and the sanctuary of Allah is the things that He has forbidden. In the body there is a piece of flesh which, if it is sound, the entire body will be sound, and if it is corrupt, the entire body will be corrupt. It is the heart.” (Muslim)

5. What Allah has prescribed in the relationship between husband and wife is sufficient and there is no need for anything else. Allah has not forbidden anything except that which is harmful.

We should realize that in His complete wisdom, when Allah forbids something (in this case, intercourse in the back passage), He also forbids the things that lead to it, because when a person does the things that lead to a haram action, his heart becomes attached to it and finds himself torn between either doing the evil action or keeping away from it, and suffering because of that. So he does not keep away from haram in such a way that he has peace of mind, nor does he fall into it and fulfill his evil desires. Usually in such cases a person ends up doing what he thought that he would never do of major sins that doom a person to Hell and ruin all his affairs, both religious and worldly, and ruin his life and take away all blessings from his wealth and children, as a result of his moving far away from his Lord and transgressing His sacred limits and not caring about the fact that Allah is watching him in all his affairs. The wise man is the one who does not take lightly those things that lead to disaster in his religious commitment which is the capital of his wealth and comes before his worldly interests.

The Muslim should understand the true nature of things and what they lead to. He should not be affected by what the devil makes attractive to him or think of evil actions as insignificant, lest that cause him to become one of the losers. He has to fear Allah his Lord in secret and in public, and remember that Allah can see him and knows his intentions and what he does, as He says: “Allah knows the fraud of the eyes, and all that the breasts conceal” (Ghafir 40:19)
We should also remember that what is with Allah is better and more lasting, and that the Hereafter and the delights that it contains is better for him than this world, and that the reward for being patient in refraining from evil actions is Paradise as wide as the heavens and the earth, in which is whatever the heart could desire of perfect pleasures that are free of all evils.”