Muwatta Malik, Book 20, Hadith 169 (USC-MSA)

Yahya related to me from Malik from Nafi that Abdullah ibn Urnar used to say, "The least thing that is acceptable as a sacrificial animal is a camel or a cow."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ ‏{‏مَا اسْتَيْسَرَ مِنَ الْهَدْى‏}‏ بَدَنَةٌ أَوْ بَقَرَةٌ ‏.‏

sunnah.com

also referenced under: Book 20, Hadith 871 (In-Book)