Khuzayma bin Thabit reported,

He said that a man asked the Messenger of Allah ﷺ about approaching women in their backsides.The Messenger of Allah ﷺ replied, "It is permissible."Then he called the man back, or ordered him to be called, and when he was brought, the Prophet ﷺ asked, "What did you say? Which of the two openings, or which of the two passages, or which of the two folds did you mean? If you meant approaching her vagina from behind, then yes. But if you meant approaching her anus, then no. Allah is not shy of the truth. Do not approach women in their backsides."

Azami 6362

Sahih

عن خزيمة بن ثابت

قال إن سائلا سأل رسول الله ﷺ عن إتيان النساء في أدبارهن. فقال رسول الله ﷺ: حلال ثم دعاه، أو أمر به، فدعي فقال: كيف قلت في أي الخربتين، أو في أي الخرزتين، أو في الخصفين؟ أ من دبرها في قبلها فنعم، أم من دبرها في دبرها فلا، إن الله لا يستحي من الحق، لا تأتوا النساء في أدبارهن.

الأعظمي 6362صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Marriage
Chapter
Rights and duties of the Muslim woman, and good treatment of her
Section
Approaching a woman in her front passage however one wishes, if one avoids approaching from behind
Numbering
Jami al-Kamil 6362 (2nd edition)
Azami 6362