Musnad Ahmad 25:336

Al-Qasim ibn Malik Abu Ja'far narrated to us, from Hanzalah al-Sadusi, from Shahr ibn Hawshab, from Ibn Abbas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) prayed the Eid [prayer] as two rak'ahs, in which he did not recite anything except the Mother of the Book (Al-Fatihah), and he did not add anything to it."