the Ma'mum (a person being led by an Imam in Prayer) utters Takbirat-ul-Ihram before the Imam, their Salah is not correct. This opinion is based on the Hadith in which the Prophet (peace be upon him) said:
“The Imam is appointed to be followed, so when he says the Takbir (saying: "Allahu Akbar [Allah is the Greatest]"), you should say it; and do not say the Takbir until he has said it.”
However, if the Ma’mum realizes it and repeats Takbirat-ul-Ihram after the Imam says it, their Salah will be correct.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.