Berakhot

But what of tefillah which is a thing with which the congregation is engaged, and yet we have learnt: If he was standing reciting the tefillah and he suddenly remembered that he was a ba'al keri he should not break off, but he should shorten [each blessing]. Now the reason is that he had commenced; but if he had not yet commenced, he should not do so? — Tefillah is different because it does not mention the kingdom of heaven.  But what of the grace after meals in which there is no mention of the sovereignty of heaven, and yet we have learnt: AT MEALS HE SAYS GRACE AFTER, BUT NOT THE GRACE BEFORE? — [Rather the answer is that] the recital of the Shema' and grace after food are Scriptural ordinances, whereas tefillah is only a Rabbinical ordinance. Rab Judah said: Where do we find that the grace after meals is ordained in the Torah? Because it says: And thou shalt eat and be satisfied and bless.  Where do we find that a blessing before studying the Torah is ordained in the Torah? Because it says: When I proclaim the name of the Lord, ascribe ye greatness to our God.  R. Johanan said: We learn that a blessing should be said after studying the Torah by an argument a fortiori from grace after food; and we learn that grace should be said before food by an argument a fortiori from the blessing over the Torah. The blessing after the Torah is learnt a fortiori from the grace after food as follows: Seeing that food which requires no grace before it  requires a grace after it, does it not stand to reason that the study of the Torah which requires a grace before it should require one after it? The blessing before food is learnt a fortiori from the blessing over the Torah as follows: Seeing that the Torah which requires no blessing after it  requires one before it, does it not stand to reason that food which requires one after it should require one before it? A flaw can be pointed out in both arguments. How can you reason from food [to the Torah], seeing that from the former he derives physical benefit? And how can you reason from the Torah [to food], seeing that from the former he obtains everlasting life? Further, we have learnt: AT MEALS HE SAYS THE GRACE AFTER BUT NOT THE GRACE BEFORE?  — This is a refutation. Rab Judah said: If a man is in doubt whether he has recited the Shema', he need not recite it again. If he is in doubt whether he has said 'True and firm', or not, he should say it again. What is the reason? — The recital of the Shema' is ordained only by the Rabbis, the saying of 'True and firm' is a Scriptural ordinance.  R. Joseph raised an objection to this,  'And when thou liest down, and when thou risest up'. — Said Abaye to him: That was written with reference to words of Torah. We have learnt: A BA'AL KERI SAYS MENTALLY, AND SAYS NO BLESSING EITHER BEFORE OR AFTER. AT MEALS HE SAYS THE GRACE AFTER BUT NOT THE GRACE BEFORE. Now if you assume that 'True and firm' is a Scriptural regulation, let him say the blessing after the Shema'? — Why should he say [the blessing after]? If it is in order to mention the going forth from Egypt, that is already mentioned in the Shema'! But then let him say the former, and he need not say the latter?  — The recital of Shema' is preferable, because it has two points.  R. Eleazar says: If one is in doubt whether he has recited the Shema' or not, he says the Shema' again. If he is in doubt whether he has said the Tefillah or not, he does not say it again. R. Johanan, however, said: Would that a man would go on praying the whole day! Rab Judah also said in the name of Samuel: If a man was standing saying the Tefillah and he suddenly remembered that he had already said it, he breaks off even in the middle of a benediction. Is that so? Has not R. Nahman said: When we were with Rabbah b. Abbuha, we asked him with reference to disciples who made a mistake and began the weekday benediction on a Sabbath, whether they should finish it, and he said to us that they should finish that blessing! — Are these cases parallel? In that case one  is in reality under obligation,  and it is the Rabbis who did not trouble him out of respect for the Sabbath, but in this case he has already said the prayer. Rab Judah further said in the name of Samuel: If a man had already said the Tefillah and went into a synagogue and found the congregation saying the Tefillah, if he can add something fresh, he should say the Tefillah again, but otherwise he should not say it again. And both these rulings are required.  For if I had been told only the first, I should have said, This applies only to [a case where he said the Tefillah] alone and [is repeating it] alone