All this is yours, but this one is mine I have a vague notion of a quote that says something about offering everything freely, but reserving some small part as absolutely off-limits.

I seem to recall the phrasing was short and sweet. I can't put my finger on it, but I'm quite sure I've seen allusions to this used as a literary device, perhaps humorously. Is there something like that in the Bible? How does it go and what was it about?
Genesis 2:16-17 is the closest that comes immediately to my mind. (quoted from NIV):


  16And the LORD God commanded the man, “You are free to eat from any tree in the garden; 17but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die.”