From ‘Amr ibn Maimun who said: I saw, in the age of ignorance [jahiliya], a monkey, and a group of monkeys gathered over her; she had committed adultery, and they stoned her. And I stoned her along with them.
Sahih Al-Bukhari, The book of the commendable acts of the Helpers [Ansar]

From ‘Amr ibn Maimun who said: I was in Yemen out on a promontory with a herd that belonged to my family, when a monkey came by with a female monkey and rested his head on her arm. Then a monkey younger than him came and winked at her, and she took her arm out slowly and gently from under the first monkey’s head, and followed him. And he had sex with her with me watching. Then she returned and put her arm gently under the first one’s cheek. He woke up panicking, smelled her, and screeched, and the monkeys gathered around, with him screeching and gesturing towards her with his hand. And all the monkeys went around right and left, and brought that monkey, the one I saw, and they dug a pit for both of them, and stoned them. And so I witnessed a stoning among non-humans.