Musnad Ahmad 1153:397

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah informed us, from Khalid ibn Yazid, from Ibn Shihab al-Zuhri, from Urwah, from Aisha: "That the Messenger of Allah, peace and blessings of Allah be upon him, used to say the takbir in the two Eids seven times in the first rak'ah, and five in the last one, excluding the takbirs of the bowing (ruku')."