Shiva Purana
नादो मध्यमुखादेवं पंचधाऽसौ विजृंभितः । एकीभूतः पुनस्तद्वदोमित्येकाक्षरो भवेत् १९ ।
The Nāda (mystical sound) came from the middle face. Thus the complete set cropped up in five-fold form. Then all of them united in the syllable of “Om”.
english translation
nAdo madhyamukhAdevaM paMcadhA'sau vijRMbhitaH । ekIbhUtaH punastadvadomityekAkSaro bhavet 19 ।
hk transliteration by Sanscript