Musannaf Ibn Abi Shayba 3:2181

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Mughira informed us, from Ibrahim, who said: If he reaches the Imam and he (the Imam) had already performed a forgetfulness prostration (sajdat al-sahw) before that, then he should perform the prostration with the Imam. Then he should complete what he missed.