Musannaf Ibn Abi Shaybah 4764 — A man who misses a rakah with the imam

Al-Hasan was asked about a man who misses one rak'ah with the imam; when the imam ends the prayer with salam, he thinks that he had joined the imam from the beginning of the prayer, so he stands up to pray voluntary prayer. Al-Hasan said: When he has brought voluntary prayer into an obligatory one, his prayer is spoiled.