Mudgalapurana 3.18.24

3.18.24

saṃnyāsibhiśca viprendra catuṣpādā viśeṣataḥ |
japtavyā saiva gāyatrī svasvāśramavibhāgataḥ ||

And by renouncers, O best of brahmins, especially the four-footed [Gāyatrī] / is to be recited, that very Gāyatrī according to the division of their respective stages of life.