Sayyiduna Jabir (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Sacrifice only a two-toothed animal (musinnah); in case of necessity, a jadh'ah of sheep may be sacrificed."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19128