Musannaf Ibn Abi Shayba 3:1382

Ghundar narrated to us, from Shu'bah, from Abu al-Fayd, who said: I heard Abu Shaybah al-Mahri narrating from Mu'adh that he said regarding a man praying behind the Imam: If he hears his recitation, he should recite "Say, 'He is Allah, [who is] One,'" and "Say, 'I seek refuge in the Lord of mankind,'" and "Say, 'I seek refuge in the Lord of daybreak.'" Shu'bah said: "Or something similar to them." And if he does not hear the recitation, he should recite, and he should not annoy those on his right and those on his left.