Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.1.19

14.2.1.19

indrāśvineti indro vai yajñasya devatā sā yaiva yajñasya devatā tāmevaitatprīṇātyaśvinetyaśvināvevaitadāhāśvinau vā etadyajñasya śiraḥ pratyadhattāṃ tāvevaitatprīṇāti tasmādāhendrāśvineti

With, 'O Indra and ye Asvins!'--for Indra is the deity of the sacrifice, and he thus pleases him who is the deity of the sacrifice; and 'Ye Asvins' he says, because the Asvins at that time restored the head of the sacrifice, and it is them he thus pleases: therefore he says, 'O Indra and ye Asvins!'