Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.4.27

4.3.4.27

athāśvaḥ | vajro vā aśvo vajramevaitatpurogāṃ kurute yamaloke me 'pyasaditi vai yajate yo yajate tadyamaloka evainametadapitvinaṃ karoti taṃ yamāya brahmaṇe 'dadāt

Then the Horse--for the horse is a thunderbolt: he thereby makes the thunderbolt the leader. And, moreover, he who sacrifices, sacrifices with the hope 'May there be a place for me in Yama's world!' He thus makes him a sharer in Yama's world. This he gave to Yama, the Brahman.