Too much of a good thing
Excess may do you harm.
The Oxford Dictionary of Quotations lists this phrase as proverbial and dates it from the late 15th century. Th earliest example that I can find of it in print is from Shakespeare's As You Like It, 1600:
ROSALIND: Why then, can one desire too much of a good thing?
Come, sister, you shall be the priest and marry us.
Give me your hand, Orlando. What do you say, sister?
See other phrases and sayings from Shakespeare.