Musnad Abu Ya'la al-Mawsili 109:325

Ubayd Allah ibn Umar narrated to us, Hisham ibn Abd al-Malik narrated to us, Zuhayr narrated to us, Abu al-Zubayr narrated to us, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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 has reached six months) from the sheep."