What is the ruling on the prayer of someone who forgot the opening Takbir (Takbirat al-Ihram): should he complete his prayer and then repeat it, or should he stop it and repeat it immediately?

If a worshiper forgets the opening takbir (Allahu Akbar) or doubts whether he said it, he must say it immediately and count whatever he has caught up with after that takbir. So, whoever says the takbir after the first rak'ah (unit of prayer) with the Imam has passed, he must make it up after the Imam finishes his prayer. And whoever says the takbir during the third rak'ah, he has missed two rak'ahs and must make them up after the Imam finishes his prayer. This applies to someone who does not suffer from waswasah (obsessive whispering). As for someone who suffers from waswasah, he should consider himself to have said the takbir at the beginning of the prayer and not make up anything, in order to defy Satan and combat his whispers.