Sayyiduna Jabir (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Sacrifice only a two-toothed animal (musinnah); if there is hardship (that is, such an animal cannot be found), then you may sacrifice a young lamb (jadha'ah) of sheep."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10153