Shiva Purana
अर्द्धमात्रा महेशानि बिन्दुनादस्वरूपिणी ॥ वर्णनीया न वै चाद्धा ज्ञेया ज्ञानिभिरेव सा ॥ १८ ॥
O goddess, this half mātrā is in the form of Bindu and Nāda. This cannot be described directly. It is known only by the wise.
english translation
arddhamAtrA mahezAni bindunAdasvarUpiNI ॥ varNanIyA na vai cAddhA jJeyA jJAnibhireva sA ॥ 18 ॥
hk transliteration by Sanscript