Reference for Bekhorot 12:14

שמעון העמסוני היה דורש כל את ואת שבתורה כיון שהגיע {דברים ד } לאת ה' אלהיך תירא פירש

His pupils, thereupon, said to him: 'Rabbi, every eth which you have expounded, - what will become of them? ' He replied to them: 'Just as I have received reward for interpreting every eth, so I shall receive reward for abstaining'.

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