Sayyiduna al-Hasan bin Ali (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) commanded us to wear the best clothing we could find, to apply the best perfume we could find, and to sacrifice the fattest animal we could find - a cow on behalf of seven people, and a camel on behalf of ten - and to proclaim the takbir aloud while tranquillity and dignity are upon us."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2743