Wutong yíyér tiān Xiàzh qi
Interpretation: From the falling of a plane tree leaf, we know that autumn is coming.
That is, seeing the big from the small and inferring the coming of autumn from the dead leaves.
Source: "Huai Nan Zi Shuo Shan": "See the big from the small, and you will know the old when you see a leaf fall." Volume 24 of "Peaceful Magnolia" quotes "A leaf falls and you know the autumn on earth".