The Rambam in Mishneh Torah (Brachot, 1:15) write :
כָּל הַמְבָרֵךְ בְּרָכָה שֶׁאֵינָהּ צְרִיכָה הֲרֵי זֶה נוֹשֵׂא שֵׁם שָׁמַיִם לַשָּׁוְא וַהֲרֵי הוּא כְּנִשְׁבָּע לַשָּׁוְא וְאָסוּר לַעֲנוֹת אַחֲרָיו אָמֵן
That is :
Whoever recites a blessing unnecessarily takes the name of God in vain and is like one who swears in vain.
And it's seems that it been understood by Poskim as a real Deoraïta, torah-interdiction. However, I don't understand this idea as :
- "The forms of all the blessings were established by Ezra and his Court" (1:5), so how can we say that the Torah would make a difference between "simple" pronouncing the name of God and in a bracha, if this concept was not existing ?
- Why would it be allowed to say the name of God in a Zmira/Kina/etc but not in a bracha ? A bracha could be considered as a "gratitude" (shvach') for Hashem, even if I don't eat/drink anything !