Muwatta Imam Malik Book of Hajj, Hadith 167

Yahya related to me from Malik from Jafar ibn Muhammad from his father that Ali ibn Abi Talib used to say, "The least difficult thing acceptable as a sacrificial animal is a sheep."

وَحَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ جَعْفَرِ بْنِ مُحَمَّدٍ، عَنْ أَبِيهِ، عَنْ عَلِيِّ بْنِ أَبِي طَالِبٍ، كَانَ يَقُولُ {مَا اسْتَيْسَرَ مِنَ الْهَدْىِ} شَاةٌ .