Vaisesika Sūtra 3.1.13

भूतो भूतस्य ॥१३॥

bhūto bhūtasya ॥13॥

[भूतः (bhūtaḥ) - the one that has become; भूतस्य (bhūtasya) - of the existent (thing).]

The one that has become from an existing thing.