Mudgalapurana 3.18.23

3.18.23

tataḥ sā tripadā vipra gāyatrī ca tataḥ punaḥ |
oṅkāro vidhinā nityaṃ japaḥ saṅkriyate dvijaiḥ ||

Then that three-footed Gāyatrī, O brahmin, and then once again / the syllable Oṃ. Recitation is always performed thus according to rule by the twice-born.