Muwatta Malik Hadith 168

Yahya related to me from Malik that he had heard that Abdullah ibn Abbas used to say, "The least difficult thing acceptable as a sacrificial animal is a sheep." Malik said, "That is what I like most out of what I have heard about the matter, because Allah, the Blessed and Exalted, says in His Book, 'O you who trust, do not kill game while you are in ihram. Whoever of you kills it intentionally, there shall be repayment the like of what he has slain, from livestock, as shall be judged by two men of justice among you, a sacrificial animal which will reach the Kaba, or food for poor people, or the equivalent of that in fasting,' (Sura 5 ayat 95) and a sheep is one of the animals which is judged to be acceptable as a sacrifice. Allah has called it a sacrificial animal, and there is no dispute among us about the matter. How, indeed, could anyone be in doubt about the matter? A sheep is the kaffara for anything which does not reach the extent of something for which a camel or a cow would be the kaffara, and the kaffara for something which does not reach the extent of something for which a sheep would be the kaffara is fasting, or feeding poor people."