Shatapatha Brahmana 3.3.2.6

3.3.2.6

tadāyatsu vācayati | eṣa te gāyatro bhāga iti me somāya brūtādeṣa te traiṣṭubho bhāga iti me somāya brūtādeṣa te jāgato bhāga iti me somāya brūtāccandonāmānāṃ sāmrājyaṃ gaceti me somāya brūtādityekaṃ vā eṣa krīyamāṇo 'bhikrīyate candasāmeva rājyāya candasāṃ sāmrājyāya ghnanti vā enametadyadabhiṣuṇvanti tametadāha candasāmeva tvā rājyāya krīṇāmi candasāṃ sāmrājyāya na badhāyatyathetya prāṅgupaviśati

While they go there, he (the Adhvaryu) makes (the sacrificer) say the text (Vâg. S. IV, 24), 'Say thou, for me, unto Soma, "This is thy gâyatrî-part (bhâga)!" Say thou, for me, unto Soma, "This is thy trishtubh-part!" Say thou, for me, unto Soma, "This is thy gayatî-part!" Say thou, for me, unto Soma, "Obtain thou the supreme sovereignty of the names of metres!"' Now, when he (king Soma) is bought, he is bought for one (destination)--for the sovereignty of the metres, for the supreme sovereignty of the metres; and when they press him, they slay him: hereby now he says to him, 'It is for the sovereignty of the metres, for the supreme sovereignty of the metres that I buy thee, not for slaying thee.' Having gone there, he sits down (behind the Soma) with his face towards the east.