What is the ruling on a congregant who says the opening takbir before the imam?

Following the Imam is obligatory; it is not permissible to precede him, nor to lag excessively behind him. If someone says the takbīrat al-iḥrām (the opening Allahu Akbar) before his Imam, his prayer is not valid by consensus (ijmāʿ), and he must repeat it. As for one who coincides with the Imam in saying the takbīr, the majority opinion is that his prayer is not valid, and this is the correct view.