Musannaf Ibn Abi Shayba 3:3320

Hushaym narrated to us, he said: Mujalid informed us, from al-Sha’bi, from Masruq, who said: ‘Abdullah used to teach us the takbir in the two Eids: nine takbirs, five in the first [rak'ah] and four in the last [rak'ah]. And he would join the two recitations together.