Ruling on the prayer of one who forgets the opening takbir

Q: If a person forgets the opening takbir, should he continue in the prayer until he finishes and then repeat it, or should he break off and then repeat it?

A: If one forgets the opening takbir or is in doubt about it, he must say the takbir immediately and act according to what he catches after the takbir. If he says the takbir after the first rak'ah of the imam's prayer has passed, he considers that the first rak'ah has passed for him, so he makes it up after the imam's taslim. If he repeats the takbir in the third rak'ah, he considers that two rak'ahs have passed for him, so he performs two rak'ahs after the taslim of the prayer. This is if he does not have waswasah (whispers of doubt). However, if he is a person prone to waswasah, he considers that he said the takbir at the beginning of the prayer and does not make up anything, in defiance of Shaytan and in fighting his whispers. And praise be to Allah.