When is the Takbirat al-Ihram pronounced, what is the ruling on the prayer of one who omits it out of ignorance, and what must he do?

The opening Takbir (Takbirat al-Ihram) is one of the pillars of prayer (Salah); the prayer is not valid without it. It is the very first act by which the prayer begins, by saying "Allahu Akbar" (God is the Greatest). It is the key to prayer and makes unlawful for the worshipper what was permissible before it. It is to be recited while standing, facing the Qibla, and it is recommended to raise the hands to the level of the shoulders when uttering it. Whoever intentionally or unintentionally omits it, their prayer is not valid. Whoever omits it out of ignorance of its obligation must repeat the prayer if there is still time remaining. If the time for the prayer has passed, it is more cautious to repeat it.