An equivalent English saying for "Those who are more humble and calm are more knowledgeable"

Question

How would you normally convey the following message throgh a fixed saying:

Those who are quiet and seem to be more humble, may belong very deep knowledge or even very strong feelings.

Note: It's not about talkativeness at all. It alluds to a belief which says: "The wiser you get, the more quiet you will become."

• Example:
  Although he dosn't speak much, he's a very knowledgeable person and as the saying goes.............................

How would you fill in the blank?

I can just translate what we say in out language, but I don't know what to say as a native would say:

• The most fruitful branch is nearest to the ground.

Thank you.

Answer 1

Still waters run deep.

Originally the proverb implied that silent people are dangerous because their motives cannot be read—they may be harboring hostile intentions. Today, however, the proverb is usually understood to mean that silent people have hidden depths of insight and emotion—they should not be dismissed merely because they don't parade their knowledge and feeings.