Reference for Berakhot 40:7

ובברכת המזון: פשיטא מהו דתימא הואיל וכתיב (שמות טז, ח) בתת ה' לכם בערב בשר לאכל ולחם בבקר לשבע כמצות עשה שהזמן גרמא דמי קמ"ל:

And Grace after meals. This too is evident ! But thou mightest say that since it is written thereby, "When the Lord shall give you in the evening flesh to eat and in the morning bread to the full" (Exod. xvi. 8), it is like a command the observance of which depends upon a certain point of time [and women are exempt]; therefore it informs us [that it is obligatory].

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