Shiva Purana
अस्मात्पंचाक्षराज्जाता मातृकाः पंचभेदतः । तस्माच्छिरश्चतुर्वक्त्रात्त्रिपाद्गाय त्रिरेव हि २२ ।
From the five-syllabled mantra the five mothers were born. The Śiromantra is born of that. The three-footed Gāyatrī also came out of the four faces.
english translation
asmAtpaMcAkSarAjjAtA mAtRkAH paMcabhedataH । tasmAcchirazcaturvaktrAttripAdgAya trireva hi 22 ।
hk transliteration by Sanscript