Hadith 508690
Chapter 20: Hajj - كتاب الحج
وَحَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ جَعْفَرِ بْنِ مُحَمَّدٍ، عَنْ أَبِيهِ، عَنْ عَلِيِّ بْنِ أَبِي طَالِبٍ، كَانَ يَقُولُ {مَا اسْتَيْسَرَ مِنَ الْهَدْىِ} شَاةٌ .
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."
Reference Muwatta Malik 508690
USC-MSA web (English) reference : Book 20, Hadith 167