The importance of pronouncing words in the right way and using grammatically correct language is emphasized in several verses of the Rigveda. The Vedic sages insist that while reciting the hymns if correct language is not used and if the words are not correctly pronounced then the gods are not appeased. This means that a sense of purity in grammar, pronunciation, and vocabulary had developed as early as the twelfth century B.C.E. For instance, the hymn 26 of Mandala 7, which is attributed to Vasiṣṭha Maitravaruni, opens with a verse which notes that Indra is pleased with sacred formulations or correct chanting of hymns. Here’s a translation of the verse 7.26.1 (The Rigveda, by Stephanie W. Jamison and Joel P. Brereton, OUP, 2014):
“Soma, unpressed, does not exhilarate Indra, nor do pressings unaccompanied by sacred formulations (exhilarate) the bounteous one.
For him I beget a hymn that he will enjoy, a newer manly one, so that he will listen to us.”