Anal Sex is a Major Sin And An Unnatural Act- [Severe Warning And Rebuke By Imaam Ibnul Qayyim (rahimahullaah)]
In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Imaam Ibnul Qayyim [rahimahullaah] said: The Quraish and the Ansaar used to approach their women from the back [i.e. sexual intercourse in the vagina whilst the wife is not lying down on her back], so the Yahood rebuked them for that. Then Allaah [The Mighty and Majestic] revealed: [نِسَاؤُكُمْ حَرْثٌ لَّكُمْ فَأْتُوا حَرْثَكُمْ أَنَّىٰ شِئْتُمْ – Your wives are a tilth for you, so go to your tilth (i.e. 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]. [2:223]
In the Saheehayn [Bukhaari and Muslim] it has been reported from Jaabir [radiyallaahu-anhu] who said: The Yahood used to say: If one has sexual intercourse with his wife from the back, she will deliver a squint-eyed child. So Allaah [The Mighty and Majestic] revealed: [نِسَاؤُكُمْ حَرْثٌ لَّكُمْ فَأْتُوا حَرْثَكُمْ أَنَّىٰ شِئْتُمْ – Your wives are a tilth for you, so go to your tilth (i.e. 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]. 
The vagina is the place of seeking after children. As for the anus, it has not been uttered on the tongue of a single Prophet among the Prophets that it is permissible [to have sexual intercourse in the anus]. Imaam Abu Daawud [rahimahullaah] reported in his Sunan from Abu Hurairah [radiyallaahu-anhu] that the Messenger [sallal-laahu-alayhi-wasallam] said: Cursed is the one who has sexual intercourse with his wife in the anus. 
If it is the case that Allaah has prohibited sexual intercourse through the vagina due to an apparent harm [i.e. at the time of menstruation or post natal bleeding], then what about the area [i.e. the anus] which is a place for the removal of harmful things [i.e. faeces] permanently.
The woman has a right to be fulfilled by the husband during sexual relations; therefore, to perform sexual intercourse with her through the anus is violation of this right. Anal intercourse neither fulfils her sexual desires nor does it reach its objective. Also, neither is the anus naturally prepared for this act nor was it created for that purpose; rather the vagina is the natural place prepared for sexual relations.
Sexual intercourse through the anus is harmful for the man; likewise it is greatly harmful for the woman because it is something very strange and far removed from innate natural human disposition.
It is from the greatest causes of punishment and blessings being taken away because it brings about Allaah’s Anger and Curse. It also deprives the person of shyness because shyness gives life to the heart [i.e. makes it hate and stay away from evil (and) repugnant acts]. However, if the heart is deprived of this shyness, it starts beautifying repugnant acts and declaring good acts repugnant, and thus corruption becomes strengthened and dominant.
[Source: بدائع التفسير الجامع لما فسَّره الإمام ابن القيم الجوزية Abridged and slightly paraphrased. Vol 1. Page: 176-177]
 Bukhaari. Number: 4528
 Hadeeth declared Hasan by Imaam Albaani (rahimahullaah) in his checking of Sunan Abu Dawud. Hadeeth Number: 2162. Page: 327. Publisher: Maktabah Al-Ma-aarif