Vaisesika Sūtra 2.1.27

परिशेषाल्लिंगमाकाशस्य ॥२७॥

pariśeṣālliṁgamākāśasya ॥27॥

[परिशेषात् लिङ्गं आकाशस्य॥
pariśeṣāt = from the residue, from the remainder; liṅgaṁ = characteristic, sign, indication; ākāśasya = of space॥]

The residue (after the effect, i.e. sound) is the distinguishing mark of the space.