If a person intentionally recites the other surah before Sūrat al-Ḥamd, his prayers are invalid. If he mistakenly recites the other surah before Sūrat al-Ḥamd and realises his mistake while he is reciting it, he must stop reciting it, recite Sūrat al-Ḥamd, and then recite the other surah from the beginning.