Torah Temimah cites Bava Kamma 105b.
“And he swore falsely” — we have learned the penalty; from where the warning? Scripture says “you shall not swear falsely” (Kedoshim).
Linkage of onesh to azharah for sheker — penalty and prohibition paired.
False-oath liability rests on the explicit prohibition in Kedoshim. The asham structure is grounded in both the warning and the penalty.