Shiva Purana
आमध्याह्नजपेद्विप्रो गायत्रीं वेदमातरम् । पंचाक्षरादिकान्मंत्रान्पश्चादासप्तिकं जपेत् ६८ ।
The Brahmin (vipra) should chant the Gayatri mantra, the mother of the Vedas, at midday. After that, he should chant the Panchakshara mantra and other mantras for a week.
english translation
AmadhyAhnajapedvipro gAyatrIM vedamAtaram । paMcAkSarAdikAnmaMtrAnpazcAdAsaptikaM japet 68 ।
hk transliteration by Sanscript