Hazrat Qatadah says that Hazrat Anas missed one rak‘ah with the imam, so he stood up and performed nafl prayers. Then he remembered that rak‘ah, so he performed it and did two prostrations.

Hazrat Hakam says that if a person misses a rak‘ah with the imam and, forgetting it, becomes engaged in nafl prayers, then when he remembers, he should break the nafl prayers and perform that rak‘ah. The narrator says that I think he also said that he should perform two sajdahs.

Hasan was asked about the person who misses one rak‘ah with the imam, and when the imam says salam, he thinks that he had joined the imam from the beginning, so he says salam and starts offering nafl prayers. Hasan said: When he included the nafl prayer in the obligatory prayer, his prayer was invalidated.