Māṇḍūkī Śikṣā 106

106

rephaṃ svarodaye vidyādṛkāraṃ vyañjanodaye
svaravyañjanayormadhye rephameva vinirddiśet

One should recognize an r-sound when followed by a vowel, and the sound *ṛ* when followed by a consonant; / between a vowel and a consonant, one should designate only an r-sound.