Abu Dawud Book 9, Hadith Number 2791.
Chapter : On the age of the sacrificial animal.
Narated By Jabir : The Apostle of Allah (PBUH) said: Sacrifice only a full grown animal unless it is difficult for you, in which case sacrifice a lamb.
Narated By Jabir : The Apostle of Allah (PBUH) said: Sacrifice only a full grown animal unless it is difficult for you, in which case sacrifice a lamb.