A man said the Takbeerah of Ihraam before the Imaam then he continued to follow the Imaam until he finished the prayer. What should he do in such a case?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

Whoever says the Takbeerah of Ihraam before the Imam he is praying behind, his prayer is invalid. He must repeat the prayer according to the majority of scholars including Maaliki, Shaafi‘i and Hanbali; regardless whether he did it deliberately or out of forgetfulness. This view is supported by the saying of the Messenger of Allaah : "The Imam is to be followed; if he says Takbeer then say Takbeer, and if he bows then bow.” [Muslim and others]

Allaah Knows best.