‏الأضاحي : Hadeeth No:4836 Jabir reported Allaah's Messenger (sallAllaahu alayhi wa sallam) as saying: Sacrifice only a grown-up animal, unless it is difficult for you, in which case sacrifice a ram (of even less than a year, but more than six months' age)....

‏الإيمان : Hadeeth No:270 It is narrated on the authority of Abu Huraira that the Messenger of Allaah (sallAllaahu alayhi wa sallam) said: Islam initiated as something strange, and it would revert to its (old position) of being strange. so good tidings for the stranger....