Māṇḍūkī Śikṣā 104

104

ṛkārapratyaye rephaḥ saṃyuktaḥ śaṣasaiḥ saha
ādyastatra kramo jñeyo na paro vodhito budhaiḥ

When an r-sound is followed by the sound *ṛ* and conjoined with *ś*, *ṣ*, or *s*, / the first is known there as subject to doubling; the following is not taught by the wise.