A traveler performed the ‘Ishaa’ prayer but stood for the third Rak‘ah and completed all four Rak‘ahs. Must he make a prostration afterwards?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The traveler who performed the ‘Ishaa’ prayer, intending to shorten it, but forgot and stood for the third Rak‘ah and completed the prayer out of Sahw (forgetfulness), must prostrate for that Sahw before the Salaam (saying "As-Salaamu ‘alaikum wa rahmat Allaah" to end the prayer) because he made an addition to the prayer.

But if he stood for the third Rak‘ah intentionally or he forgot but then continued intentionally after he remembered, without the instance of something requiring him to complete the prayer, like the intention to end his travel or otherwise, then his prayer is invalid because he intentionally added to it that which does not belong to it.

An-Nawawi said: "If the person praying by himself intended to shorten his prayer but then stood for a third Rak‘ah, then if something that necessitated completing the prayer occurred, like following the resident Imaam, or being resident, or reaching the port of residence in a ship, and he stood for that reason, then he did what he must do. But if something like that did not happen and he stood intentionally, then his prayer is invalid, without any scholarly disagreement, because he intentionally added to his prayer what does not belong to it. The same applies for a resident who stands for a fifth Rak‘ah or someone praying a non-obligatory prayer but stands for an additional Rak‘ah without previously changing his intention. If he stood out of forgetfulness and then he remembers, he must return to sitting, prostrate for the Sahw, and end his prayer with Salaam. Further, if he wanted to complete the prayer after remembering this addition, he must return to sitting, and then rise again to complete the prayer."