Musannaf Ibn Abi Shayba 3:2305

Abu Bakr narrated to us, he said: Yahya ibn Sa'id narrated to us, from Sa'id, from Qatadah, from Sa'id ibn al-Musayyib, regarding a man who catches a portion of his prayer with the Imam. He said: He performs what he caught, and he does not perform the two prostrations.