Musannaf Ibn Abi Shayba 3:237

Hammad ibn Khalid narrated to us, from Abd al-Rahman ibn Abi al-Mawali, from Umar ibn Abi Muslim, who said: Urwah ibn al-Zubayr used to say: "If one of you comes while the Imam is prostrating, let him prostrate with the people, but he should not count it [as a rak'ah]."