Gershon Dubin asked:
Dear Rabbi Kornfeld,
From where do we know that ein shevuah chala al shevuah, or the rule of "mushba ve'omed"?
The Kollel replies:
The Gemara in Shevuos (25-27) learns this from the Pasuk, "l'Hara O leHeitiv" (Vayikra 5).