Shiva Purana
यदुच्चनीचस्वरितैःस्पष्टास्पष्टपदाक्षरैः ॥ मंत्रमुच्चारयेद्वाचा वाचिको ऽयं जपस्स्मृतः ॥ २६ ॥
The muttering of the mantra with high, low or middle accentuation, the words and the letters being clear or otherwise, is called the verbal japa.
english translation
yaduccanIcasvaritaiHspaSTAspaSTapadAkSaraiH ॥ maMtramuccArayedvAcA vAciko 'yaM japassmRtaH ॥ 26 ॥
hk transliteration by Sanscript