If I remember correctly, Buddha wanted to verify his insight after his sit under the Bodhi tree. He took a leaf and reversed the cause-and-effect leading to him holding that leaf in order to check something. Can anyone point to the story?
In the case of Gauthama the Buddha, there is no description about a leaf. The Buddha did review dependent origination, direct, reverses and direct & reverse order, as in the other answers.
But there are cases of Pacceka Buddha's who have been enlightened by seeing a withered leaf.
Two other Jātaka tales report the attainment of Paccekabodhi on seeing a withered leaf fall. In these cases, too, it is clearly an external condition that is responsible for the break-through to awakening.
At that instant Darīmukha seeing the garden now empty came and sat on the royal seat in the garden. A withered leaf fell before him. In it he came to see the principles of decay and death, grasped the three marks of things, and making the earth re-echo with joy he entered on paccekabodhi.
But when the king was gone, Sonaka returned and sat on the stone seat, and so it was that a withered leaf of a sāl tree fell from its stalk in front of him, and on seeing it he cried, "Even as this leaf, so will my body fall into decay," and acquiring supernatural insight by reflecting on the impermanence of all things he attained to the state of a paccekabuddha, and at this very instant his characteristic as a layman vanished, and the marks of an ascetic became visible, and saying, "There is no more re-birth for me," in the utterance of this aspiration he set out for the cave of Nandamūla.
I think you're referring to a section in the first of the seven weeks after enlightenment:
In the middle watch of the night the Exalted One thoroughly reflected on "The Dependent Arising" in reverse order thus: "When this cause does not exist, this effect is not; with the cessation of this cause, this effect ceases.
With the cessation of Ignorance, Conditioning Activities cease.
With the cessation of Conditioning Activities (Relinking) Consciousness ceases.
With the cessation of (Relinking) Consciousness, Mind and Matter cease.
With the cessation of Mind and Matter, the six Spheres of Sense cease.
With the cessation of the Six Spheres of Sense, Contact ceases.
With the cessation of Contact, Feeling ceases.
With the cessation of Feeling, Craving ceases.
With the cessation of Craving, Grasping ceases.
With the cessation of Grasping, Becoming ceases.
With the cessation of Becoming, Birth ceases.
With the cessation of Birth, Decay, Death, Sorrow, Lamentation, Pain, Grief, and Despair cease.
How the Lord Buddha verified his awakening he told his first later disciples, the story delivered in the Mahavagga here.
...“Vision arose, insight arose, discernment arose, knowledge arose, illumination arose within me with regard to things never heard before: ‘ … has been comprehended, ...abandoned, ...experienced, ...developed’...
My person does not think that an Awakened would use a leave, matter, for his situations verification. So it might have been a folk or fake story, if having been touched by such.
(Note that this is not given for trade, exchange, stacks of stories and entertaining with it, but as a door for release from bonds and interest)