Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.4.23

3.3.4.23

sa vai dvirūpo bhavati | dvidevatyo hi bhavati devatayorasamade kṛṣṇasāraṅga syādityāhuretaddhyenayo rūpatamamiveti yadi kṛṣṇasāraṅgaṃ na vindedatho api lohitasāraṅga syāt

It (the victim) is two-coloured, because it belongs to two deities: 'For the sake of concord between the two deities let it be a black-spotted (buck)!' they say; 'for that is most like those two (gods).' If he be unable to obtain a black-spotted buck, it may be a red-spotted one.