What is the ruling on someone who utters the Taslim (salutation of peace) with their tongue while still performing the prayer, and did not turn their head before the Imam’s Taslim?

Greeting intentionally before the Imam invalidates the prayer. However, if it is done forgetfully, it does not invalidate it, provided the worshiper remedies it by returning to the prayer and greeting after their Imam. If they greet, thinking their Imam has greeted, they must return to the prayer with their intention, wait for their Imam, and greet with them.