Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.2.16

3.4.2.16

atha yāstaddevāḥ | juṣṭāstanūḥ priyāṇi dhāmāni sārdhaṃ samavadadire tatsārdhaṃ saṃjaghne tatsāmābhavattasmādāhuḥ satyaṃ sāma devajaṃ sāmeti

Now what favourite forms and desirable powers the gods then laid together, all that was wrought together and became the Sâman: wherefore they say, 'The Sâman is the truth, the Sâman is born of the gods.'