Muhammad bin Ali (may Allah have mercy on him) said: I was with Muhammad bin Ka'b al-Qurazi (may Allah have mercy on him) when a man came and said: O Abu Amr, what do you say about (approaching) a woman in her backside? He said: This is a shaykh of Quraysh, ask him, meaning Abdullah bin Ali bin al-Sa'ib (may Allah have mercy on him). He said that Abdullah had not heard anything about it. He said: O Allah, it is filth, even if it were lawful. Then Abdullah bin Ali met Amr bin Uhayhah bin al-Julah, and he was asked: Have you heard anything about approaching a woman in her backside? He said: I heard Khuzaymah bin Thabit al-Ansari (may Allah be pleased with him), whose testimony the Messenger of Allah (peace and blessings be upon him) made equal to the testimony of two men, say that a man came to the Messenger of Allah (peace and blessings be upon him); then he mentioned the rest of the hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14113