Tantra Sāra 1.005
तद्रूपभेदाः पञ्चाशन्मूर्तयो मम चापराः। पञ्चाशद्वर्णवाच्यास्ता वर्णास्तारार्णभेदिताः ॥५॥
tadrūpabhēdāḥ pañcāśanmūrtayō mama cāparāḥ। pañcāśadvarṇavācyāstā varṇāstārārṇabhēditāḥ ॥5॥
[tadrūpabhēdāḥ = their difference in terms of forms; pañcāśanmūrtayō = are fifty in forms; mama = mine; cāparāḥ = are varied; pañcāśadvarṇavācyāstā = they are conveyed by 50 letters of the alphabets; varṇāstārārṇabhēditāḥ = they originate from the eight sounds that correspond to pranava; ]
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':
