Vaisesika Sūtra 2.1.30

शब्दलिङ्गा विशेषाद्विशेषलिङ्गाभावाच्च ॥३०॥

śabdaliṅgā viśeṣādviśeṣaliṅgābhāvācca ॥30॥

[शब्द लिङ्गा विशेषात् विशेष लिङ्ग अभावात् च॥
śabda = the word/sound; liṅgā = distinguishing mark; viśeṣāt = because of the specificity; viśeṣa liṅga = special distinguishing mark; abhāvāt = absence; ca = and/even;]

The sound is the special distinguishing mark (of the space), because of the absence of other special distinguishing marks.