Atri Smriti 2.11

2.11

upāṃśustucalajjihvāddhadadaśanacchada īritaḥ
adharoṣṭhavibhāgo vā viśvāsopāṃśulakṣaṇaḥ
nirvikāreṇa vakreṇa manasā mānasaḥ smṛtaḥ

Whispered is declared to be with moving tongue and slight motion of the lips, / or separation of the lower lip; breathing is the mark of whispered; / with an unmoved face, through the mind alone, it is handed down as mental.