Musnad Ahmad 33:233

Hassan narrated to us, Zuhair narrated to us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not slaughter anything except a musinnah (an animal that has reached the age of maturity), unless it is difficult for you, then you may slaughter a jadha'ah (a lamb that is at least six months old) from the sheep."