Atharvaveda 11.3.36

11.3.36

[4.5]a) tátaś cainam anyáyā jihváyā prā́śīr yáyā caitáṃ pū́rva ŕ̥ṣayaḥ prā́śnan |
[4.5]b) jihvā́ te mariṣyatī́ty enam āha |
[4.5]c) táṃ vā́ aháṃ nā́rvā́ñcaṃ ná párāñcaṃ ná pratyáñcam |
[4.5]d) agnér jihváyā |
[4.5]e) táyainaṃ prā́śiṣaṃ táyainam ajīgamam |
[4.5]f) eṣá vā́ odanáḥ sárvāṅgaḥ sárvaparuḥ sárvatanūḥ |
[4.5]g) sárvāṅga evá sárvaparuḥ sárvatanūḥ sáṃ bhavati yá eváṃ véda ||36|| [5]

If thou hast eaten it with another tongue than that with which the ancient seers ate this, thy tongue will die: thus one says to him; it verily [have] I not [eaten] coming hither, nor retiring, nor coming on; with Agni's tongue, therewith have I etc. etc.