Musannaf Ibn Abi Shayba 3:3343

Khalid ibn Makhlad narrated to us, he said: Ibrahim ibn Isma'il ibn Abi Habibah narrated to us, from Dawud ibn al-Husayn, from Abu Sufyan, from Abu Sa'id al-Khudri, who said: The Takbir in the two Eids is seven and five; seven in the first [rak'ah] before the recitation, and five in the last [rak'ah] before the recitation.