It was narrated from Khuzaimah bin Thabit:
That the Messenger of Allah (ﷺ) said: "Allah is not too shy to tell the truth," three times. "Do not have intercourse with women in their buttocks." (DE)
English reference (DE): Vol. 3, Book 9, Hadith 1924 (DE)
Arabic reference (DE): Book 9, Hadith 1999 (DE)