Harith narrates from Sayyiduna Ali (may Allah be pleased with him): "Reciting aloud on the two Eids is from the Sunnah, and going out on the two Eids to the open ground (jabbanah) is from the Sunnah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة العيدين / 6196