Musnad Abu Ya'la al-Mawsili 110:135

'Abd al-A'la ibn Jabir narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from 'Amr ibn Abi 'Amr, from 'Ikrimah, from Ibn 'Abbas, that the Prophet (peace be upon him) said: "Whoever you find having sexual intercourse with an animal, kill him and kill the animal."