Māṇḍūkī Śikṣā 19

19

udāttaścānudāttaśca svaritaḥ pracitastathā
caturvidhaḥsvarodṛṣṭaḥ svaracintāviśāradaiḥ

Acute accent, grave accent, circumflex accent, and monotone accent: / tone is seen as fourfold by those skilled in the investigation of tones.