Musannaf Ibn Abi Shayba 3:2149

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from Abd al-Malik, from Ata, regarding the man who enters [the prayer] with the Imam and [the Imam] makes a mistake (sahw): He said: The prayer of the Imam suffices him, and there is no [obligation of] sahw upon him.