A traveler who joined a congregation and completed the four rak'ahs, having forgotten he was a traveler: is his prayer valid?

Question : A traveler entered the mosque at the time of the iqamah, so the people prayed a four-rak'ah prayer, and he completed the four rak'ahs, forgetting that he was a traveler, and he did not remember until he was in the prayer. Is his prayer valid?

Answer : It is valid. No, his prayer is valid, because the stronger position is that shortening the prayer is not obligatory.