Śatapatha Brāhmaṇa ("of a hundred paths") 6.3.2.1

6.3.2.1

hasta eṣābhrirbhavatyatha paśūnabhimantrayate | etadvā eṣu devā anveṣiṣyantaḥ purastādvīryamadadhustathaivaiṣvayametadanveṣiṣyanpurastādvīryaṃ dadhāti

The spade is still in his hand, when he addresses the beasts. For when the gods at that time were about to search (for Agni) in these (animals) they placed their vigour in front; and in like manner does this one, now that he is about to search in these (animals), place his vigour in front.