Ammar, the freed slave of Banu Hashim, narrates from Ibn Abbas (may Allah be pleased with them both) that he would say seven takbirs in the first rak'ah, then recite, and say five takbirs in the second rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة العيدين / 6181