Hafs bin Umar bin Sa'd al-Qaraz relates, through his father and his paternal uncles, that Sa'd al-Qaraz (may Allah be pleased with him) was the mu'adhdhin of the people of Quba'; then Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) moved him to Madinah and made him his own mu'adhdhin. The Sunnah in the prayer of Eid al-Adha and Eid al-Fitr is that the imam says seven takbirs in the first rak'ah before the recitation, and five takbirs in the second rak'ah before the recitation.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5430