Bhagavatapurana 2.5.25

2.5.25

tāmasādapi bhūtādervikurvāṇādabhūn nabhaḥ /
tasya mātrā guṇaḥ śabdo liṅgaṃ yaddraṣṭṛdṛśyayoḥ //

25. Out of the Tamasa Ahankara called Bhutadi which began to change was evolved Akasa; and of Akasa the subtle form and special characteristic is Sabda (Sound)—(i.e., from the subtle sabda the gross Akasa is produced). Sabda is that which indicates (leads to a knowledge of) the seer and the seen (i.e., the person that knows or understands and the thing that is known).