משנה על יומא 8:1

One who eats terumah unwittingly must repay its value plus a fifth, whether he eats it or drinks it, or anoints himself with it, or whether the terumah is clean or unclean. [If he eats the added fifth] he must pay its fifth, and a fifth of that fifth. He may not repay with terumah, but rather with tithed hullin, which becomes terumah. And whatever may be repaid in its place also becomes terumah and if the priest wishes to forego [the fine], he cannot do so.