Yājñavalkya Śikṣā 21

21

svasthaḥ praśānto nirbhīto varṇānuccārayedbudhaḥ
nābhyā hanyānna nirhanyānna gāyenna ca kampayet

composed, tranquil, and fearless, the wise person should pronounce the speech sounds. / He should not strike with the navel, nor expel [breath] violently, nor sing, nor tremble.