Shiva Purana
एकाक्षरांस्तथा मंत्राञ्जपेदक्षरकोटितः । ततः परं जपेच्चैव सहस्रं भक्तिपूर्वकम् 1-11-५० ।
If the mantra consists of only one syllable it shall first be repeated a crore times and thereafter a thousand times with great devotion.
english translation
ekAkSarAMstathA maMtrAJjapedakSarakoTitaH । tataH paraM japeccaiva sahasraM bhaktipUrvakam 1-11-50 ।
hk transliteration by Sanscript