Shatapatha Brahmana 5.5.4.15

5.5.4.15

atha yadāśvino bhavati | aśvinau vā enamabhiṣajyatāṃ tatho evainameṣa etadaśvibhyāmeva bhiṣajyati tasmādāśvino bhavati

And as to why there is (a victim) for the Asvins,--it was the Asvins who cured him; and in like manner does he (the priest) now cure him through those same Asvins: that is why there is (a victim) for the Asvins.