Musnad Ahmad 30:1558

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, al-A'raj narrated to us, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Takbir in the two Eids is seven [times] before the recitation, and five [times] after the recitation."