Māṇḍūkī Śikṣā 120

120

saṃyogasya paraṃ svārthaṃ paraṃ saṃyoganāyakam
saṃyuktasya tu varṇasya na svaraṃ pūrvamiṣyate

The latter of a conjunction has its own value; the latter is the ruler of the conjunction; / but a vowel preceding a conjoined speech sound is not desired.