Musannaf Ibn Abi Shayba 3:3203

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Yazid ibn Ibrahim narrated to us: Al-Hasan was asked about a man who broke his ablution on Friday, so he went to perform ablution, and he came back while the Imam had already prayed. He said: He shall pray four [rak'ahs].