A man who misses a rakah with the imam

Qatadah said: Sayyiduna Anas (may Allah be pleased with him) missed one rak'ah with the imam. He stood up and prayed voluntary prayer; then he remembered the rak'ah, so he prayed it and performed two prostrations.

Al-Hakam said: If a man missed a rak'ah with the imam and, forgetting it, became occupied with voluntary prayer, then when he remembers he should break off the voluntary prayer and pray that rak'ah. The narrator said: I think he also said: And he should perform two prostrations.

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.