Musannaf Ibn Abi Shayba 18:1143

Abu Bakr narrated to us, he said: Waki’ narrated to us, from ‘Abd al-‘Aziz ibn Husayn, from Yahya ibn Yahya al-Ghassani, he said: A man burned straw in a pen of his, and a spark of fire flew out until it burned something belonging to his neighbor. He said: So I wrote regarding it to ‘Umar ibn ‘Abd al-‘Aziz. He wrote back to me that the Messenger of Allah (may Allah bless him and grant him peace) said: "The injury caused by an animal is exempt (from liability), and I consider the fire to be exempt (from liability)."