Musannaf Ibn Abi Shayba 3:3436

Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan, who said: He should say the takbir with him in what he catches of it, and he should make up what he missed, and he should say the takbir in it like the takbir of the Imam in the second rak'ah.