"لَا تَذْبَحُوا إِلَّا مُسِنَّةً, إِلَّا أَنْ يَعْسُرَ عَلَيْكُمْ فَتَذْبَحُوا جَذَعَةً مِنَ اَلضَّأْنِ"
Jabir (RAA) narrated that Allah's Messenger (ﷺ) said: "Sacrifice only a full-grown animal unless it is difficult for you, in which case you should sacrifice a (six to ten month old) sheep." Reported by Muslim.