Māṇḍūkī Śikṣā 18

18

dvitīyaṃ svaritaṃ prāhuḥ ṣaṣṭhaḥ pracita ucyate
uccaṃ vidyānniṣādantu nīcaṃ ṣaḍjamudāhṛtam

They declare the second to be the circumflex accent; the sixth is called the monotone accent; / one should know niṣāda as high, and ṣaḍja is declared to be low.