Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.2.12

3.8.2.12

athottānam paśum paryasyanti | sa tṛṇamantardadhātyoṣadhe trāyasveti vajro vā asistatho hainameṣa vajro 'sirna hinastyathāsinābhinidadhāti svadhite mainaṃ hiṃsīriti vajro vā asistatho hainameṣa vajro 'sirna hinasti

Thereupon they turn the victim over so as to lie on its back. He (the Adhvaryu) puts the (other) stalk of grass thereon, with, 'O plant, protect!' for the knife is a thunderbolt, and thus that thunderbolt, the knife, does not injure it (the victim). He then applies the edge of the knife to it (and cuts through it) with, 'Injure it not, O blade!' for the knife is a thunderbolt, and thus that thunderbolt, the knife, does not injure it.