score:4
From the Pali canon perhaps it's the Simsapa Sutta (see here and here).
The main message I get from that story is a bit the opposite of what you suggested: i.e. not that "The Dharma is actually all the leaves in all the trees", rather that the central doctrine is only the four noble truths (stress; origin of stress; cessation of stress; path of practice leading to the cessation of stress).
I think there are other suttas in which he said, "I have taught one thing and one thing only, i.e. dukkha and the cessation of dukkha.