Tantra Sāra 1.005
तद्रूपभेदाः पञ्चाशन्मूर्तयो मम चापराः। पञ्चाशद्वर्णवाच्यास्ता वर्णास्तारार्णभेदिताः ॥५॥
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':
