Sahih Muslim 12223
In-book no 4946حَدَّثَنَا 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).