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