In his delight he lifted up his voice and brayed, but then everyone knew him, and his owner came up and gave him a sound cudgelling for the fright he had caused. And shortly afterwards a Fox came up to him and said: "Ah, I knew you by your voice."
How did the Fox know that it was the Ass ?
By the Ass' voice
By the cudgelling
the Ass' owner told him
People told him