Shiva Purana
दिवाऽतीते च गायत्रीं शतं नित्ये क्रमाज्जपेत् । आदर्शाहात्पराऽतीते गायत्रीं लक्षमभ्यसेत् 1-13-३० ।
If the omission is for ten days or more, Gāyatrī must be repeated for a hundred thousand times as atonement.
english translation
divA'tIte ca gAyatrIM zataM nitye kramAjjapet । AdarzAhAtparA'tIte gAyatrIM lakSamabhyaset 1-13-30 ।
hk transliteration by Sanscript