Reference for Shevuot 56:4

שתיהן בשוגג פטור

R. Mari said: We have also learnt thus [in a Mishnah]: Four vows did the Sages permit: vows of urging. vows of hyperbole. vows made unwittingly, and vows accidentally unfulfilled. Vows made unwittingly: how? 'Konam [this loaf to me], if I ate or drank [today]', and he remembered that he had eaten or drunk; '[konam this loaf to me,] if I eat or drink [today]', and he forgot, and ate or drank, he is permitted [to eat that loaf]; and it was taught with reference to this: just as vows made unwittingly are permitted, so oaths made unwittingly are permitted. <br>

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