It is allowed to have intercourse with one's wife in her anus according to Imam Malik, al-Bukhari and Ibn Umar.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. Having anal intercourse with one's wife is a repulsive and detested matter. It is forbidden in Shariah according to the majority of Salaf from Sahabah, Thabieen, and the Imams who came after them. The person who practices it is deprived from Allah's Sight. The Prophet (Sallallahu Alaihi wa Sallam) said: Allah does not Look at the one who has intercourse with his wife in her rectum. Imam Ahmad , Abu Dawood and al-Nasa'e reported from Abu Hurairah that the Prophet (Sallallahu Alaihi wa Sallam) said: Accursed is the one who has anal intercourse with his wife. Imam Malik 's opinion in this matter is just as the opinion of other Imams, i.e. having sex in anus is forbidden. Everyone who says otherwise about him (Imam Malik ) fabricates lies against him. Imam Ibn Katheer reported in his Tafseer from Israeel bin Rooh who said, I asked Malik about making sex with wives in their anuses. He replied, 'you are but Bedouins, does the act of tilling happen other than in the place of tilth? Do not exceed the vagina. I said, people say that you allowed that? He answered, they fabricate lies against me, they fabricate lies against me, and he kept repeating this sentence'. This opinion is proved from Malik and the same opinion is held by Imams Abu Hanifah , al-Shafi'e , Ahmad bin Hanbal and all his followers; prior to them, the prohibition is also reported, through sound narration, by Sayeed bin al-Musayyih , Abu Salamah , Ikramah , Tawoos , 'Ataa , Sayeed bin Jubair , Urwah bin al-Zubair , Mujahid bin Jaber , al-Hasan and many others. Salaf also strongly condemn it, furthermore some of them even considered it as committing Kufr. [1:347] What is reported from Ibn Umar in this concern is not more than merely a misconception. Imam Ibn al-Qayyim thoroughly refuted it, and said what is reported from Ibn Abbas in this concern is the same as, what was reported from Ibn Umar . Ibn Abbas said regarding this verse: {Your wives are as a tilth unto you; so approach your tilth when or how ye will; but do some good act for your souls beforehand and fear Allah, and know that ye are to meet him (in the Hereafter), and give (these) good tidings to those who believes. }[2:223]. It means having sex with wives while they are laying on their bellies or their backs. In all cases pleasure should be sought only through vagina. The same opinion is also reported from Ibn Umar . Imam Nasa'e reported from Abu al-Nadhar that he asked al-Nafi (the great Imam and known pupil of Ibn Umar ) and said, It is being reported from you that you allowed having sex in anus. He said, they uttered slander against me. I will inform you for the reason of this misunderstanding. Once Ibn Umar was reciting some Qur'an while I was there, he passed by the verse: {Your wives are as a tilth unto you; so approach your tilth when or how ye will;…}[2:223] and said, O Nafi' ! Do you know what this verse means? We the clan of Quraish used to expose women in an uncomely matter during sex. When we came to Medinah and married the women of Medina and tried to enjoy with them as we used to enjoy with our women, they refused it since they used to have sex by only one way. In this regard Allah revealed the verse: {Your wives are as a tilth unto you; so approach your tilth when or how ye will;…}[2:223]. Then he ( Ibn Umar ) explained that the verse means having sex in vagina from the backside. We do not know anyone who ever reported from Imam al-Bukhari that he allowed anal sex with one's wife. He is far above saying such a matter. We sum up that having sex in anus is totally forbidden and a major sin. It is also forbidden according to the four Imams and others. Therefore, do not pay any attention to those who allowed it based on their whims. They do not have any sound basis for their opinion, neither from the Qur'an nor from the Sunnah. Allah knows best.