Ruling on the prayer of one who prostrated only one prostration out of forgetfulness

Q: An imam prayed the 'Asr prayer, and during the last rak'ah he prostrated only once, then sat for the final tashahhud and said the taslim. After that, some of the worshippers alerted him that he had not prostrated twice in the last rak'ah. He then stood up and prayed a complete rak'ah with them, sat for the tashahhud, said the taslim, and performed sujud al-sahw. Is this manner correct?

A: Yes, this is what is prescribed. If the imam forgets a prostration and says the taslim, then he remembers or is alerted, he stands up and performs a rak'ah, then completes the prayer, then says the taslim, and then performs sujud al-sahw after the taslim, which is better. The ruling for the one praying alone is the same. If he performs sujud al-sahw before the taslim, there is no harm, but after the taslim is better.