(a) Ata said: Ibn Abbas (may Allah be pleased with them both) used to say twelve takbirs in the two Eids: seven in the first rak'ah and five in the second.
This chain is sahih (authentic).
(b) It has also been narrated in this report from Abd al-Malik bin Abi Sulayman: "thirteen takbirs, seven in the first rak'ah and six in the second," as though he counted the takbir of standing up as well.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة العيدين / 6180