It is narrated from Nafi' (may Allah have mercy on him) that Abdullah bin Umar (may Allah be pleased with them both) used to say: By "whatever is easy from the sacrificial animal" is meant the sacrifice of a camel or a cow.
Hadith Referenceمسند الإمام الشافعي / كتاب الحج / 835
Hadith Takhrijاسناده صحيح اخرجه مالك في المؤطا ، الحج، باب ما استيسر من الهدى والبيهقي : 24/5 ۔ وفي المعرفة السنن والآثار له ۔ (374) وابن ابی شیبه (12778) ،(12781) ، (12788)۔