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. (JA)
English reference (JA): Book 12, Hadith 1392 (JA)
Arabic reference (JA): Book 12, Hadith 1351 (JA)