If, on account of ignorance, a person adds or omits something that is an obligatory rukn, the prayer is invalid. As for omitting something that is obligatory, but not a rukn, by someone who is inculpably ignorant – such as someone who trusts the words of a reliable person or a credible manual of Islamic rulings (risālah), and afterwards he realises that the person or the manual was wrong – this does not invalidate the prayer.
Furthermore, in the event that a person does not know the relevant ruling, even if it is his own fault [for not knowing it], and he recites Sūrat al-Ḥamd and the other surah of ṣubḥ, maghrib, and ʿishāʾ prayers in a whisper, or he recites Sūrat al-Ḥamd and the other surah of ẓuhr and ʿaṣr aloud, or he performs ẓuhr, ʿaṣr, and ʿishāʾ prayers as four rakʿah prayers when he is a traveller, in all these cases, his prayer is valid.