Dutiyadārukkhandhopama sutta: The Simile of the Tree Trunk (2nd)

At one time the Buddha was staying near Kimibilā on the bank of the Ganges River. Seeing a large tree trunk being carried along by the current, he addressed the mendicants: “Mendicants, do you see that large tree trunk being carried along by the current of the Ganges River?”

“Yes, sir,” they replied. … When this was said, Venerable Kimbila said to the Buddha:

“But sir, what’s the near shore and what’s the far shore? What’s sinking in the middle? What’s getting stranded on high ground? What’s getting taken by humans or non-humans? What’s getting caught up in a whirlpool? And what’s rotting away?” …

“And what, Kimbila, is rotting away? It’s when a mendicant has committed the kind of corrupt offense for which no rehabilitation is possible. This is called ‘rotting away’.”