Reference for Shevuot 48:4

ומאן דפטר דלית ליה כי אמר איסור מוסיף בחדא חתיכה בשתי חתיכות לא אמרי'

If you will, you may say, the Tanna mentions only that which does not come within the category of absolution, but an oath which comes within the category of absolution he does not mention. - But [he mentions] holy food! - Well, we have established that it refers to a firstborn.

Jastrow