Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1145
Narated By 'Aisha, Ummul Mu'minin : The Apostle of Allah (pbuh) would say the takbir (Allah is most great) seven times in the first rak'ah and five times in the second rak'ah on the day of the breaking of the fast and on the day of sacrifice (on the occasion of both the 'Id prayers, the two festivals).