What is the ruling on one who doubts the recitation of Al-Fatiha after commencing the Surah, then returns to a verse within it and completes his prayer? Is his prayer valid?

If you complete reciting Al-Fatihah, believing in its correctness, then doubt an error in one of its verses after starting the next Surah, this doubt has no effect, and you should not return to correct the suspected error. The prayer is not invalidated by returning to Al-Fatihah to correct a suspected verse, and you do not need to repeat it. Repeating Al-Fatihah or a verse from it is permissible if it is due to doubt about the mastery of the recitation. And whoever errs in a word from Al-Fatihah should repeat only that word and what follows it.