Musannaf Ibn Abi Shayba 3:4742

Abu Khalid al-Ahmar narrated to us, from Ash'ath, from Abu Ishaq, from al-Harith, from Ali, regarding the man who misses a rak'ah or two rak'ahs with the Imam: He said, "He recites during the silence of the Imam." And al-Hasan said the like of it.