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

                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.
    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.