Reference for Shevuot 6:5

קסבר רבי ישמעאל לאו שאין בו מעשה לוקין עליו

[for lashes]; just as a vain oath which is necessarily in the past [being untrue the moment it is uttered, is attended by the penalty of lashes], so is a false oath in the past [attended by the penalty of lashes].

Jastrow