Harith bin Amr (may Allah be pleased with him) said: I came to the Prophet (peace and blessings be upon him) at Arafat or Mina, and people had gathered around him (peace and blessings be upon him). A man asked him (peace and blessings be upon him) about the atirah, and he (peace and blessings be upon him) said: "Whoever wishes may slaughter an animal, and whoever does not wish need not; and whoever wishes may slaughter the young of a goat, and whoever does not wish need not." And concerning sheep he said: "In sheep, its sacrifice," and Abu Ma'mar explained it to us by pointing with his index finger: one.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19343