The Bodhisatta was once an ascetic. He had lived in the Himalayas with his young son ever since his wife died. One day during a heavy rain, a wet and cold monkey saw the two of them in their leaf hut staying comfortable next to a fire. He knew that if he asked to come in and warm himself, he would be shooed away, so he took the clothes from a dead ascetic and sat by their hut hoping to trick them into inviting him in.
When the son saw him, not knowing he was a monkey, he told his father there was an old ascetic there and they should let him come sit by the fire. The Bodhisatta took a look and explained to his son that humans do not have faces like that; it was a monkey. And monkeys were not allowed in because they would defile their home. The Bodhisatta threw a log at the monkey and it fled into the forest.
In the Lifetime of the Buddha
The monkey was an earlier birth of one of the Buddha’s disciples who claimed to dedicate himself to seeking salvation, but was known by other disciples to be dishonest. The Buddha told them this story so they knew that this dishonest disciple had also been dishonest in the past.
The son was an earlier birth of the Buddha’s son.