He was traveling but he forgot and stood up for a third rak’ah

A man was travelling and he stood up for a third rak’ah in a prayer that he had intended to shorten. Does he have to sit back down or should he offer the prayer in full? What does he have to do?.

Praise be to Allah, and blessings and peace be upon the Messenger of Allah:

It is better for him to sit back down, because he started the prayer on the basis that he intended to pray two rak’ahs, so he should pray two rak’ahs and no more. And he should do the prostration of forgetfulness after the salaam. But if he continues and offers the prayer in full, there is nothing wrong with that.