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." (FR)
English reference (FR): Vol. 3, Book 9, Hadith 1924 (FR)
Arabic reference (FR): Book 9, Hadith 1999 (FR)