Short answer
The mainstream Islamic scholarly position is that anal intercourse is prohibited between husband and wife.
Islamic Guidance
What Islam teaches
Prophetic narrations warn against this practice, and the four major Sunni schools of Islamic jurisprudence regard anal intercourse as prohibited.
Practical Guidance
What you can consider
Muslim couples should follow established Islamic guidance rather than assuming that marriage makes every form of sexual activity permissible.
Sources
Sunan Abi Dawud; Jami' al-Tirmidhi; Sunan Ibn Majah.