(a) Ubaydullah bin Abdullah bin Utbah bin Mas'ud (may Allah have mercy on him) says: It is the Sunnah for the imam, on the day of Eid al-Fitr and Eid al-Adha, when he sits on the pulpit before the sermon, to say nine takbirs, and seven when he stands. Then he should supplicate, and after that say takbir as much as is decreed for him.
(b) Ubaydullah (may Allah have mercy on him) says: He should say nine takbirs in succession when he stands in the first sermon, and seven takbirs in succession when he is in the second sermon.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة العيدين / 6216