Praise be to Allah.
Allaah says (interpretation of the meaning):
“Your wives are a tilth for you, so go to your tilth (Have sexual relations with your wives in any manner as long as it is in the vagina and not in the anus), when or how you will, and send (good deeds, or ask Allaah to bestow upon you pious offspring) for your ownselves beforehand. And fear Allaah, and know that you are to meet Him (in the Hereafter), and give good tidings to the believers (O Muhammad).” [al-Baqarah 2:223]
From the word harth (tilth) we understand that what is permissible is only in the vagina (the front passage), especially because this is what will produce children. The semen that is planted in the womb from which offspring come is likened to the seeds which are planted in the ground, from which vegetation comes, as both of them are substances from which something else is produced.
The phrase translated as when or how you will means, in any manner you wish, from behind or from the front, sitting or with the wife lying on her back or on her side, so long as it is in the place of tilth (i.e., the vagina, the place from which a baby is born).
The poet said:
The wombs are lands for us to till; we have to plant the seeds and whatever grows is up to Allaah.
It was reported from Khuzaymah ibn Thaabit (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Allaah is not too shy to tell you the truth: do not have intercourse with your wives in the anus.”
(Narrated by Imaam Ahmad, 5/213; a hasan hadeeth).
Ibn ‘Abbaas said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Allaah will not look at a man who has intercourse with his wife in her anus.”
(Narrated by Ibn Abi Shaybah, 3/529; narrated and classed as saheeh by al-Tirmidhi, 1165).
See Nayl al-Maraam by Siddeeq Hasan Khaan, 1/151-154.
If a man does this, his wife is not considered to be divorced as many people think, because there is no shar’i evidence at all that indicates this. But the scholars said that if a man habitually does this, his wife has the right to ask for a divorce, because he is an evildoer (faasiq) who is causing harm by his action, and also because the purpose of marriage cannot be achieved through this action. The wife has to resist this evil action and remind her husband about Allaah and about the punishment for transgressing the limits set by Allaah. If the husband repents to Allaah from this deed, there is no reason why she should not stay with him, and there is no need to renew the marriage contract. And Allaah is the Source of strength.