Proving that a specific grammar is ambiguous How can I prove that the following grammar is ambiguous:

$$ A \to AA\mid B  \\  B \to aBb\mid ab $$

I tried finding a string that can be derived in two different ways, but to no avail.
The word $ababab$ can be derived in two different ways.

If you don't see why, I suggest starting with the simpler grammar $A \to AA|a$.