Shiva Purana
स्थित्वासने जपेन्मौनी चैलाजिनकुशोत्तरे ॥ आब्राह्मं च मुहूर्ते तु गायत्री दृढमानसः ॥ १२ ॥
Standing silently on a seat on a cloth, deerskin and kusha grass one should chant the mantra. At the moment of Brahma he chanted the Gayatri mantra with a firm mind.
english translation
sthitvAsane japenmaunI cailAjinakuzottare ॥ AbrAhmaM ca muhUrte tu gAyatrI dRDhamAnasaH ॥ 12 ॥
hk transliteration by Sanscript