Musannaf Ibn Abi Shayba 3:129

Waki’ narrated to us, from Ibrahim ibn Isma’il, from al-Zuhri, from ‘Urwah ibn al-Zubayr and Zayd ibn Thabit, that they used to come while the Imam was bowing, so they would say the opening takbir for the prayer and for the rak’ah.