Yunus (may Allah have mercy on him) narrates from al-Hasan (may Allah have mercy on him) regarding the takbir of the days of Tashriq that he said: 'Allahu akbar, Allahu akbar, Allahu akbar' (Allah is the Greatest, Allah is the Greatest, Allah is the Greatest); and Ata (may Allah have mercy on him) says that he used to say the takbir three times.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة العيدين / 6281