Tantra Sāra 1.005
tadrūpabhēdāḥ pañcāśanmūrtayō mama cāparāḥ। pañcāśadvarṇavācyāstā varṇāstārārṇabhēditāḥ ॥5॥
Further variations of my forms are fifty. They are conveyed by fifty letters of the Sanskrit alphabets. They originate from the eight sounds that correspond to Pranava.
A pictorial reprasentation of how everything is derived from the Pranava, 'Om':
