Outside of a dog, a book is man's best friend. Inside of a dog, it's too dark to read.

As per research conducted by Quote Investigator, more than 25 years before its first in-print attribution to Marx, on April 5, 1973 (as "Outside of a dog, a book is man's best friend. That's because inside a dog it's too dark to read"), the first appearance of this quote—as "Outside a horse, a book is man's best friend—inside it's too dark to read"—occurred in a May 31, 1947 newspaper column, where it is attributed to jockey Ted Atkinson.