Where is this stated?

"According to the Jedi Master Zao, the soup that Anzati drank was the future, and thus, the Force. Because of this, Anzati who drank the soup of another Force-sensitive would strengthen his connection to the Force."

Which book is this from? Or novel or magazine, or whatever?

I'd guess it would be from the Star Wars Republic comics, specifically the issues dealing with Volfe Karkko.

http://www.readcomics.tv/star-wars-republic/chapter-35/5

What's Zao's obsession with ****ing soup?

XD.

I was just thinking that.

I don't know if thats really concrete evidence though.

Was Karkko confirmed to have grown in the force after eating "soup"?

Karkko says in the next issue that after drinking Quinlan's and other jedis' soup, he would be strong.
http://www.readcomics.tv/star-wars-republic/chapter-35/15

and he does have experience, since he's stated to have drunken jedi essence before.
http://www.readcomics.tv/star-wars-republic/chapter-35/17

This in addition to Tholme's reluctance to allow Vos to face Karkko in fear of having Vos's soup consumed all point to the conclusion that drinking jedi soup does make Karkko stronger.