Musannaf Ibn Abi Shayba 3:128

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Ma’mar, from al-Zuhri, from Salim, from Ibn Umar and Zayd ibn Thabit, they both said: "If a man catches up with the people while they are bowing, then a single takbir is sufficient for him."