Rajam Iyer leaned back in his seat, reminding himself of a proverb which said that if you threw a stone into a gutter it would only spurt filth in your face.
I'd never heard this before, but I like it. In context it's a metaphor for not arguing with idiots, along the same lines as "don't mud-wrestle with a pig, because you'll both get filthy but the pig will enjoy it" and "don't play chess with pigeons: they'll knock over the pieces, crap all over the board, and then strut around like they've won". But I like this one even better, and I'd like to know its source. The story suggests it's a traditional or well-known proverb, but Googling it only gave me pages about this very story.
Did Narayan invent this saying, or is it really a proverb? If the latter, what language/culture does it come from?
It might be that my Google search for a real proverb failed because this only became known in English through Narayan, and in some Indian language/culture it might indeed be well known. For what it's worth, the character Rajam Iyer is a Brahmin, reading a book in Tamil, travelling from Madras to Bangalore. (I don't know enough about Indian cultures to know which of these facts is most likely to be relevant.)