What is the Sutra where The Buddha says that he has taught only a fraction of what he knew
That sutta (or, at least, one of those suttas) is sometimes summarized as "a handful of leaves".

Simsapa Sutta: The Simsapa Leaves


  "In the same way, monks, those things that I have known with direct knowledge but have not taught are far more numerous [than what I have taught]. And why haven't I taught them? Because they are not connected with the goal, etc.