Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.7.3

4.6.7.3

tadvā etat | sahasram vācaḥ prajātaṃ dve indrastṛtīye tṛtīyaṃ viṣṇurṛcaśca sāmāni cendro yajūṃṣi viṣṇustasmātsadasyṛkṣāmābhyāṃ kurvantyaindraṃ hi sadaḥ

This, then, is the thousandfold progeny of Vâk (speech). Indra (obtained for his share) two-thirds, and Vishnu one-third: the Riks and Sâmans are Indra, and the Yagus are Vishnu. Therefore in the Sadas they perform (the Sastras and Stotras) with the Rik and Sâman, for the Sadas is Indra's own.