Musannaf Ibn Abi Shayba 3:6274

Ibn Mahdi narrated to us, from Muhammad ibn Muslim, from ‘Amr ibn Dinar, from ‘Ata’, regarding a man who catches up with the Imam during the Witr [prayer] in the prayer: He said, "He does not perform the Tashahhud." And ‘Amr ibn Dinar said, "I hold that view."