Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.40

7.3.1.40

tadāhuḥ | kathamasyaitāḥ pṛthaṅnānā yajurbhirupahitā bhavantīti mano vai yajustadidam mano yajuḥ sarvāḥ sikatā anuvibhavatyevamu hāsyaitāḥ pṛthaṅnānā yajurbhirupahitā bhavanti

As to this they say, 'How is that (sand) of his put on separately with different prayers?' Well, prayer is thought; this thought, prayer, comes to be equal to the whole sand: and thus that (sand) of his comes to be put on separately with different prayers.