This teaching is from the section The Four Noble Truths - The Matrix of the Teaching from "Noble Truths, Noble Path" by Bhikkhu Bodhi.

The Buddha explains why he teaches only a little compared to what he has not taught with the simile of the rosewood leaves.

Once, the Blessed One was dwelling in Kosambi, in the rosewood grove (grove of Simsapa trees, Dalbergia tree forest [sīsapāvana]). Then, taking a few rosewood leaves in his hand, the Blessed One addressed the bhikkhus: "What do you think, bhikkhus, which is more numerous — the few rosewood leaves that I have taken in my hand, or those in the rosewood grove above?"

"Very few, venerable sir, are the rosewood leaves that the Blessed One has taken in his hand; indeed, those in the rosewood grove above are far more numerous."

"In the same way, bhikkhus, what I have taught you is only a little compared to what I have not taught you. Why have I not taught it? Because it is not beneficial, does not relate to the fundamentals of the spiritual life, and does not lead to disenchantment (de-illusionment [nibbidā]), to fading of desire (dispassion, detachment [virāga]), to ending (cessation, termination [nirodha]), to tranquility (calmness, serenity, stillness, peace [upasama]), to direct knowing (experiential understanding [abhiññāya]), to full awakening (perfect understanding, enlightenment [sambodha]), to Nibbāna (complete cooling, letting go of everything, deathless, freedom from calamity, the non-disintegrating [nibbāna]). That is why I have not taught it.

And what have I taught, bhikkhus? 'This is suffering (mild suffering, intense suffering, discomfort, pain, disease, unpleasantness, stress, discontentment, dissatisfaction [dukkha]),' bhikkhus, I have taught; 'this is the arising of suffering (source of stress, appearance of discomfort [dukkhasamudaya]),' I have taught; 'this is the ending of suffering (ending of discontentment, cessation of distress [dukkhanirodha]),' I have taught; 'this is the way of practice leading to the ending of suffering (i.e. the noble eightfold path [dukkhanirodhagāmī]),' I have taught.

Why have I taught this? For this is beneficial, relates to the fundamentals of the spiritual life, and leads to disenchantment, to fading of desire, to ending, to tranquility, to direct knowing, to full awakening, to Nibbāna. That is why I have taught it.

Therefore, bhikkhus, effort should be made to fully understand (understand in principle, then discern in each moment and then experientially penetrate): 'This is suffering';

effort should be made to fully understand: 'This is the arising of suffering';

effort should be made to fully understand: 'This is the ending of suffering';

effort should be made to fully understand: 'This is the way of practice leading to the ending of suffering.'"

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Related Teachings:

The effects of cultivating the eightfold path are gradual (DhP 273) - The noble eightfold path is a comprehensive collection of training guidelines that one should practice for the ending of the unwholesome mental qualities and for the benefits of enlightened mental qualities shine through.

Gradual Training, Gradual Practice, and Gradual Progress (MN 107) - The gradual training guideline teaching of the Buddha is how a student starting out in the teachings of the Buddha should gradually practice in to see gradual progress towards breakthrough of fully understanding the four noble truths.

Diversity of external contacts (SN 14.9) - When one is muddle-minded, there is an arising of diverse perceptions, intentions, contacts, sensations, desires, fevers, quests, and acquisitions. Furthermore, for an uninstructed ordinary person, these mental patterns are often mistaken as aspects of the self, taken hold of, and assumed to be "who I am."