Vaisesika Sūtra 7.1.07

एकद्रव्यत्वात् ॥७॥

ekadravyatvāt ॥7॥

[एक (eka) - one; द्रव्यत्वात् (dravyatvāt) - because of the nature of substance;]

Because of the nature of being one substance.