Vaishnava Dharmashastra 55.13

55.13

sahasrakṛtvas tv abhyasya bahir etat trikaṃ dvijaḥ
mahato 'py enaso māsāt tvacevāhir vimucyate //

By repeating those three (Om, the 'words,' and the Gâyatrî every day) for a month out of the village, a thousand times, a twice-born man is purified even from a mortal sin, as a snake (is freed) from its withered skin.