It was narrated from Ibn`Abbas that the Messenger of Allah (ﷺ) said:
"Whoever has intercourse with a Mahram relative, kill him; and whoever has intercourse with an animal, kill him, and kill the animal." (RU)
English reference (RU): Vol. 3, Book 20, Hadith 2564 (RU)
Arabic reference (RU): Book 20, Hadith 2661 (RU)