Musannaf Ibn Abi Shayba 3:230

Abu Bakr narrated to us, he said: Ibn Abi ‘Adi narrated to us, from Dawud, from al-Sha’bi, who said: "If you find them prostrating, then prostrate with them, but do not count it." And Abu al-‘Aliyah said: "Prostrate with them and count it."