What is the ruling on the prayer of someone who joined the congregation in the first rak'ah, then the Imam stood for a fifth rak'ah and no one glorified him [to alert him], so he completed the prayer with the extra rak'ah, knowing that he did not perform the prostration of forgetfulness (sujud al-sahw) and no one questioned the addition after the salutation?

If the Imam stands for an extra rak'ah, the congregants who are aware of the addition should say "SubhanAllah" to him. If the Imam does not return, the preponderant opinion is that it is not permissible for the congregant to follow him in this addition. He may then leave the Imam and say the tasleem, and his prayer will be valid. If the congregant follows him while aware of the impermissibility, his prayer becomes invalid. However, if he was unaware of the impermissibility, his prayer remains valid. This addition necessitates the prostration of forgetfulness (sujud al-sahw). Even if the Imam and the congregants omit it, their prayer remains valid.