Māṇḍūkī Śikṣā 98

98

āpadyate makāro yakhoṣmasu pratyayeṣvanusvāram
na bhavati lakāre parasavarṇasparśeṣucottamāpattiḥ

The m-sound becomes an anusvāra before following y, r, and spirants; / it does not become so before l, [where] it assimilates to the following sound, and before stops there is change to the nasal [stop].