Shiva Purana
पञ्चाशद्रुद्ररूपास्तु वर्णा वर्णाध्वसंज्ञिताः ॥ अनेकभेदसंपन्नः पदाध्वा समुदाहृतः ॥ ६ ॥
The fifty letters in the form of Rudras constitute the path of letters. The path of words has many varieties.
english translation
paJcAzadrudrarUpAstu varNA varNAdhvasaMjJitAH ॥ anekabhedasaMpannaH padAdhvA samudAhRtaH ॥ 6 ॥
hk transliteration by Sanscript