It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Slaughter only a two-toothed animal (musinnah); if there is difficulty (i.e. a two-toothed animal is not found), then sacrifice a jadha'ah of sheep."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19057