The Compendium of Caraka 1.30.27

1.30.27

so 'ayamāyurvedaḥ śāśvato nirviśyate anāditvāt svabhāvasaṃsiddhalakṣaṇatvāt bhāvasvabhāvanityatvācca /

This very Āyurveda is declared to be eternal, because it has no beginning, because its characteristics are established by their very nature, and because the nature of existents is eternal.