Musnad Ahmad 33:387

Hashim and Hassan ibn Musa narrated to us, they said: Zuhair narrated to us, Abu al-Zubayr narrated to us—Hassan said: 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."