What do you say concerning a man who performed a prayer and increased its number of rak'ahs, yet his prayer was valid, and what is the evidence for that?

If a four-rak'ah prayer is missed during travel and remembered when at home, some scholars, such as Ahmad, Al-Awza'i, Dawud, and Al-Shafi'i in one of his views, state that it should be prayed as four rak'ahs. They argue that shortening the prayer (qasr) is a concession that ceases with the end of travel, and that the prayer became obligatory with its full number of rak'ahs when at home, based on the evidence "let him pray it when he remembers it." In contrast, Malik, Al-Thawri, and the اصحاب الرأي (scholars of independent reasoning) hold that it should be prayed as two rak'ahs, because only two rak'ahs were missed.