حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا أَبُو الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لاَ تَذْبَحُوا إِلاَّ مُسِنَّةً إِلاَّ أَنْ يَعْسُرَ عَلَيْكُمْ فَتَذْبَحُوا جَذَعَةً مِنَ الضَّأْنِ ‏"‏ ‏.‏

Jabir reported Allah's Messenger (PBUH) 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). Translation Not Available

Book Ref: Sahih Muslim Book 35 Hadith 4836
Web Ref:  Sahih Muslim Book 22 Hadith 1963