Mudgalapurana 3.18.15

3.18.15

vyāhṛtīnāṃ ca gāyatryāḥ ṣaḍaṅgeṣvakṣarodbhavān |
gāyatrīśiraso nyāsāṃstata ācamanaṃ caret ||

and those of the Vyāhṛtis, and of the Gāyatrī arising from the syllables on the six limbs; / having performed the placements of the head of the Gāyatrī, he should then sip water.