Rif Brachot 12b

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12b

Rav Yehuda cited Shmuel: If he has a doubt whether or not he read Shema, he does not return and read. If he has a doubt whether he said emet veyatziv, he returns and reads.
It is clear then that he holds that reading Shema is Rabbinic while emet veyatziv is Biblical.

And Rabbi Eliezer said: If he has a doubt whether or not he read Shema, he returns and reads. If he has a doubt whether or not he prayed {Shemoneh Esrei} he does not return and read.

Rabbi Yochanan said: If only a man would pray all day!

And the Sages explain that these words {of Rabbi Yochanan about praying all day - and thus saying an extra Shmoneh Esrei} are by an individual with an intent of it being voluntary, but if his intent is it being obligatory, it is forbidden. And in a congregation whether the intent is as voluntary or obligatory