Narapatijayacarya 2.2.2

2.2.2

mātṛkāyāṃ purā proktāḥ svarāḥṣoḍaśasaṃkhyān ||
a ā i ī u ū ṛ ṝ lṛ lṝ e ai o au aṃ aḥ |
teṣāṃ dvāvantimau (aṃ aḥ)tyājyau----
-catvāraśca (ṛ ṝ lṛ lṝ )napuṃsakau ||

In the phonetic matrix sixteen sounds were formerly proclaimed: / a, ā, i, ī, u, ū, ṛ, ṝ, lṛ, lṝ, e, ai, o, au, aṃ, aḥ. / Of these, the final two (aṃ, aḥ) are to be abandoned, / and the four neuter ones (ṛ, ṝ, lṛ, lṝ).