Vaisesika Sūtra 1.1.27

संयोगानां द्रव्यम् ॥२७॥

saṁyogānāṁ dravyam ॥27॥

[संयोगानाम् (saṁyogānām) - of conjunctions; द्रव्यम् (dravyam) - substance;]

The substances are formed by conjunction.