Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.4.31

4.3.4.31

athāśvam pratyeti | yamāya tvā mahyaṃ varuṇo dadātviti yamāya hyetaṃ varuṇo 'dadātso 'mṛtatvamaśīya hayo dātra edhi vayo mahyam pratigrahītra iti

And the horse he accepts with, 'Let Varuna give thee to me, Yama!' for to Yama Varuna gave it. 'May I obtain immortality! be thou a steed (hayah) to the giver, strength (vayas) to me, the receiver!'