It is narrated from Urwah (may Allah have mercy on him) that Abu Ayyub Ansari and Zaid bin Thabit (may Allah be pleased with them both) instructed Marwan to say seven takbirs in the first rak'ah and five takbirs in the second rak'ah of the Eid prayers.
Hadith Referenceمسند الإمام الشافعي / كتاب العيدين والأضاحي والاستسقاء / 487