Musannaf Ibn Abi Shayba 3:2309

Ibn ‘Ulayyah narrated to us, from Ayyub, from Abu Qilabah, who said: If a man catches one prostration of the Imam’s prayer, he should prostrate another one with it, then prostrate two prostrations after he finishes his prayer. And if he catches two prostrations, he should prostrate after he finishes his prayer.