In Following the Equator, this quote is attributed to the Pudd’nhead Wilson’s New Calendar. Those quote serves in part as a promotional tool for Mark Twain’s previous novel Pudd’nhead Wilson, published in 1894.

Pudd’nhead Wilson’s New Calendar — MT came back to Pudd’nhead and his homespun ironies a few years later, when he used more of these maxims as chapter epigraphs in Following the Equator (1897). He came back to Pudd’nhead in this way in part, obviously, because he hoped to continue to promote the novel, and in part because of the popular attention the first batch of Pudd’nhead’s sayings had received. But whatever boundary had separated Sam Clemens’ experience from Pudd’nhead’s voice is even more permeable now: it is harder than ever to attribute what Wilson says in these “new” aphorisms with his character as it is developed in the novel. So it makes sense to think of the “Pudd’nhead” role as another of the disguises (like “Mark Twain” itself) Clemens found useful as a rhetorical resource. (Mark Twain in His Time)

The whole “new” Wilson’s calendar is available over at the Mark Twain in His Time website.