All perfect praise be to Allah, the Lord of the Worlds; may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Having intercourse with wife from her back passage is haram. Rather, scholars have considered this a major sin and the righteous predecessors used to call it "Lesser Sodomy" (intercourse with a woman in her back passage).This is regardless of whether the wife agreed to this act or not.
There are many Sahih (sound) narrations which show that anal intercourse is prohibited, such as the following:
1- It was narrated from Abu Hurayrah (may Allah Be Pleased with him), the Messenger of Allaah (PBUH) said: “The one who has intercourse with his wife in her back passage has disavowed himself of that which was revealed to Muhammad (PBUH).” Related by Abu Dawood (Hadith No.3904).
2- It was narrated that Ibn ‘Abbaas (may Allah Be Pleased with them) said: The Messenger of Allaah (PBUH) said: “Allah will not look at a man who has intercourse with a woman in her back passage.” Narrated by Al-Tirmidhi (No.1165); graded as a sound narration by Ibn Daqeeq Al-‘Eid in [Al-lmaam vol.2/pp.660].
3- It was narrated that Khuzaymah Ibn Thaabit (may Allaah be pleased with him) said: The Messenger of Allaah (PBUH) said: “Allaah is not too shy to tell the truth” three times. “Do not have intercourse with women in their back passages.” [Ibn Majah No.1924)].
And there are many similar narrations. Al-Tahhaawi (may Allaah have mercy on him) said in Sharh Ma’aani al-Athaar (vol.3/pp.43): The reports concerning that reach the level of tawaatur. End quote.
Hence the views of the scholars are based on these narrations.
Al-Mawardi (may Allah have mercy on him) stated :
"Because that is the consensus of the Sahaabah. It was narrated from ‘Ali ibn Abi Taalib, ‘Abd-Allah ibn ‘Abbaas, Ibn Mas’ood and Abu’l-Darda’ (May Allah Be Pleased with them." [Al-Haawi (vol.9/pp.319].
In conclusion, whoever committed this major sin must make sincere repentance, ask Allah for forgiveness and resolve not to recommit it. However, the wife isn`t considered divorced and no expiation is payable because it is, mostly, required in case of minor sins, not major ones. And Allah The Almighty Knows Best.