Yahya related to me from Malik from Jafar ibn Muhammad from hisfather that Ali ibn Abi Talib used to say, "The least difficult thingacceptable as a sacrificial animal is a sheep." (RU)
USC-MSA web (English) reference: Book 20, Hadith 167 (RU)
Arabic reference (RU): Book 20, Hadith 869 (RU)