Reference for Shevuot 41:2

ואיבעית אימא כשם שמביא קרבן על שקר כך מביא קרבן על שוא ור"ע היא דמחייב לשעבר כלהבא

An objection was raised: What is a vain oath? Swearing that which is contrary to the facts known to man. A false oath? Swearing that which is the reverse. [Hence, a false oath is in the past tense, yet R. Johanan says, in the future.] Say, Swearing and reversing. <br>

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