Musannaf Ibn Abi Shayba 3:2315

Hushaym narrated to us, from Yunus, from al-Hasan, that he used to say regarding a man who missed a rak'ah with the Imam: When the Imam performed the taslim, he (the man) thought that he had caught the beginning of the prayer with him, so he stood up to perform a voluntary prayer. Al-Hasan said: If he enters into a voluntary prayer while he is in an obligatory prayer, his prayer is corrupted.