Hadith 1963
Chapter 2: The age of sacrificial animals - كتاب الأضاحى
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 (ﷺ) 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).
Reference : Sahih Muslim 1963
In-book reference : Book 35, Hadith 19
USC-MSA web (English) reference : Book 22, Hadith 4836