What is the ruling on leaving out al-Fatihah in prayer intentionally or by mistake?

Question:
What is the ruling if a person leaves out al-Fatihah intentionally or by mistake in the loud or quiet prayer?

Answer:
If he is a ma'mum, that does not harm him, if he is ignorant or forgetful; because it is obligatory for him, and it is dropped due to ignorance and forgetfulness, and it is dropped if he arrives while the imam is bowing, and the rak'ah suffices for him.
As for if he is an imam or praying alone, it is not dropped, meaning if he forgets it in a rak'ah, a rak'ah takes its place and he performs a second rak'ah, so if he concludes the prayer and a long interval has passed, he repeats the prayer; because it is a pillar for the imam and the one praying alone, and it is necessary, and if he forgets it in a rak'ah, the other rak'ah takes its place and he performs an extra rak'ah until he completes it.
As for the ma'mum, it is obligatory for him, and it is dropped due to mistake and forgetfulness, and it is dropped due to missing it if he arrives while the imam is bowing, he bows with him and it suffices for him, as came in the hadith of Abu Bakrah (may Allah be pleased with him) .