Sahih Bukhari Volume 007, Book 067, Hadith Number 422.
Narrated By Ibn 'Umar : That he entered upon Yahya bin Said while one of Yahya's sons was aiming at a hen after tying it. Ibn 'Umar walked to it and untied it. Then he brought it and the boy and said. "Prevent your boys from tying the birds for the sake of killing them, as I have heard the Prophet forbidding the killing of an animal or other living thing after tying them."