In chapter 11 of Hesse's "Siddhartha," the title character is experiencing great longing for the normalcy of common people's lives as he ferries them across the river. He sees men with their sons and wishes for that life for himself, rather than the journey for wisdom which he's chosen instead. This longing is described as a burning wound.
"And one day, when the wound burned violently, Siddhartha ferried across
the river, driven by a yearning, got off the boat and was willing to go
to the city and to look for his son."
Instead of doing this, Siddhartha looks down into the river and sees his reflection, recognizing in it the same pain he once saw in his father's face, when he left home.
"Had his father not also suffered the same pain for him, which he now suffered for his son?"
This is what Siddhartha asks himself. He sees the karmic cycle of his life, the pain of this father reflected in himself over his own son. He saw this repeating cycle going on in the world endlessly, in countless people. When he asks Vasudeva about this, Vasudeva is able to guide him to "hear" what the river is saying, and Siddhartha hears the oneness of things, which heals his pain of longing.
"...he heard them all, perceived the whole, the oneness, then the great
song of the thousand voices consisted of a single word, which was Om: