Musnad Ahmad 30:3076

Waki' narrated to us, he said: Sufyan narrated to us, from Suhayl ibn Abi Salih, from al-Harith ibn Mukhallid, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Cursed is he who approaches a woman in her anus."