Devibhagavatapurana 11.17.15

11.17.15

dhyāyettu manasā devīṃ mantramuccārayecchanaiḥ |
na kampayecchiro grīvā dantānnaiva prakāśayet ||

Then meditate on the Form of the Gâyatrî mentally and slowly repeat the Gâyatrî. Do not shake head, neck and while making japam, do not show your teeth. According to due rules repeat the Gâyatrî one hundred and eight times, or twenty-eight times. When unable, repeat ten times; not less than that.