Is it obligatory for a traveler who prayed 'Isha and then stood up for a third rak'ah by mistake and completed it, to perform the prostration of forgetfulness (sujood as-sahw)?

If a traveler intends to shorten the four-rak'ah prayer but mistakenly stands for the third rak'ah, he must return to the tashahhud and then perform the prostrations of forgetfulness (sujud al-sahw). If he completes the third rak'ah mistakenly, and he had already performed the tashahhud in the second rak'ah, then it is not permissible for him to perform the tashahhud in the third rak'ah; rather, he should perform the prostrations of forgetfulness. If he did not perform the tashahhud after the second rak'ah, he should perform the tashahhud in the third rak'ah and perform the prostrations of forgetfulness, due to the generality of the Prophet's saying: "For every instance of forgetfulness, there are two prostrations."