Shatapatha Brahmana 3.5.4.23

3.5.4.23

athādhiṣavaṇam parikṛttam bhavati | sarvarohitaṃ jihvā haivāsyaiṣā tadyatsarvarohitam bhavati lohinīva hīyaṃ jihvā tannidadhāti vaiṣṇavamasīti vaiṣṇavaṃ hyetat

Now the pressing-skin is cut straight all round and (dyed) red all over, for it is his (Vishnu's) tongue: the reason, then, why it is quite red, is because this tongue is, as it were, red. He lays it down with, 'Thou art Vishnu's own;' for it indeed belongs to Vishnu.