Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.1.43

7.4.1.43

tāvasyaitāvindrāgnī eva bāhū | tāvenaṃ tejasā ca vīryeṇa ca saha prapadyete sa sampratyuraḥ puruṣamākāśya yatrābhyāpnoti tadālikhyaine upadadhātyeṣa haitayorlokaḥ

Indra and Agni indeed are those two arms of his (Pragâpati's): they join him with fiery spirit and vigour. Where he (the Sacrificer) touches (the ground with his arms), whilst viewing intently the gold man with his breast close to him, there he (the Adhvaryu) makes a mark and lays down those (spoons); for that is the place of those two (arms).