Musannaf Ibn Abi Shayba 3:229

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Mu'tamir narrated to us, from Salim ibn Abi al-Dhayyal, from Qatadah, who said: "If you reach them while they are prostrating, then prostrate with them, and do not count that rak'ah."