Vijnana Bhairava Tantra
यस्य कस्यापि वर्णस्य पूर्वान्तावनुभावयेत् । शून्यया शून्यभूतोऽसौ शून्याकारः पुमान्भवेत् ॥ ४० ॥
Whoever contemplates even on the matras or letters (of Aum) from first to last, in the form of void, verily that sadhaka by meditation on the void becomes the void.
english translation
yasya kasyApi varNasya pUrvAntAvanubhAvayet । zUnyayA zUnyabhUto'sau zUnyAkAraH pumAnbhavet ॥ 40 ॥
hk transliteration by Sanscript