Yājñavalkya Śikṣā 25

25

gadgado baddhajihvaśca na varṇānvaktumarhati
prakṛtiryasya kalyāṇī dantoṣṭhau yasya śobhanau

nor a stammerer, nor a tongue-tied person is fit to utter the speech sounds. / He whose nature is auspicious, whose teeth and lips are handsome,