Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.2.1

13.2.2.1

rājā vā eṣa yajñānāṃ yadaśvamedhaḥ yajamāno vā aśdyajñamārabhatevamedho yajamāno yajño yadaśve paśūnniyunakti yajña eva ta

Verily, this--to wit, the Asvamedha--is the king of sacrifices. But, indeed, the Asvamedha is the Sacrificer, (for) the sacrifice is the Sacrificer: when he (the priest) binds victims to the horse (or, at the horse-sacrifice), he then, indeed, takes hold of the sacrifice at the sacrifice.