Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.41

7.3.1.41

tadāhuḥ | kathamasyaitāḥ sarvaiścandobhirupahitā bhavantīti yadevainā etena ṣaḍṛcena nivapati yāvanti hi saptānāṃ candasāmakṣarāṇi tāvantyetasya ṣaḍṛcasyākṣarāṇyevamu hāsyaitāḥ sarvaiścandobhirupahitā bhavanti

As to this they say, 'How does that (sand) of his come to be put on by all the metres?'--Inasmuch as he scatters it with that hymn of six verses; for as many syllables as there are in the seven metres, so many syllables there are in that hymn of six verses: thus that (sand) of his comes to be put on by all the metres.