How does one whose prayer has been preceded perform it if he catches the last rak'ah of a four-rak'ah prayer, and the imam says the taslim while he (the imam) has a post-prostration?

The madhhab (school of thought) of the majority of scholars is that the latecomer (masbuq) waits until the Imam prostrates for forgetfulness (sujud al-sahw) with him, then he stands up to complete what he missed. This is based on the Prophet's (peace be upon him) saying: "Indeed, the Imam was appointed to be followed."