Ruling on one who forgets the opening takbir and remembers during the prayer

Question:
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 the prayer and then repeat it?

Answer:
If he doubts whether he said the takbir or not, he should say it immediately and proceed with what he has caught up on after the takbir. If he repeats the takbir in the second rak'ah of the imam's prayer, he should consider that he missed the first rak'ah.
And if he repeats the takbir in the third rak'ah, he should consider that he missed two rak'ahs, in which case he makes up two rak'ahs after the prayer. This is if he is not one who is afflicted with obsessive doubts. As for one who is afflicted with obsessive doubts—having such doubts—he should not pay attention to them; he should build upon what has passed and know that he said the takbir, and praise be to Allah.