The Art of Money Getting or, Golden Rules for Making Money was originally published in 1880 by P. T. Barnum.
Part 12 Do Not Scatter Your Powers
Engage in one kind of business only, and stick to it faithfully until you succeed, or until your experience shows that you should abandon it. A constant hammering on one nail will generally drive it home at last, so that it can be clinched. When a man's undivided attention is centered on one object, his mind will constantly be suggesting improvements of value, which would escape him if his brain was occupied by a dozen different subjects at once. Many a fortune has slipped through a man's fingers because he was engaged in too many occupations at a time. There is good sense in the old caution against having too many irons in the fire at once.