Ruling of Forgetting the Takbeerat al-Ihraam

Takbeerat al-Ihraam is that a person says 'Allahu Akbar' when he begins his prayer. Prayer cannot be performed except with that. So, If a person forgets Takbeerat al-Ihraam, he comes, stands in the row, then forgets (it), begins with recitation, and prays, then his prayer is invalid. It was not performed at all. That's because prayer can only be performed/correct with Takbeerat al-Ihraam. The Prophet ﷺ said to a man whom he taught how to pray, he ﷺ said: "If you stand up for prayer, make Wudhu properly, then face the Qiblah and make Takbeer." So, it is a must to do Takbeer. The Prophet ﷺ used to always do it.