Proving ambiguity of this Context Free Grammar
The string $ababab$ has two different parse trees. In one of them the first two $ab$'s are grouped together, and in the other one, the last two $ab$'s are grouped together.

Stated differently, one parse tree corresponds to a derivation $S \Rightarrow SS \Rightarrow^* ababS \Rightarrow^* ababab$, and the other one to a derivation $S \Rightarrow SS \Rightarrow^* Sabab \Rightarrow^* ababab$. If you draw the parse trees for both derivations, you'll see that you get different parse trees.

Finally, here are actual two different leftmost derivations of $ababab$:


$S \Rightarrow SS \Rightarrow SSS \Rightarrow TSS \Rightarrow abSS \Rightarrow abTS \Rightarrow ababS \Rightarrow ababT \Rightarrow ababab$.
$S \Rightarrow SS \Rightarrow TS \Rightarrow abS \Rightarrow abSS \Rightarrow abTS \Rightarrow ababS \Rightarrow ababT \Rightarrow ababab$.


As an aside, an equivalent unambiguous grammar replaces the production $S \to SS$ by the production $S \to ST$ (or by the production $S \to TS$).