“Every time you stop a school, you will have to build a jail. What you gain at one end you lose at the other. It's like feeding a dog on his own tail. It won't fatten the dog.”

His collected works contain no riddle about dog legs, but George W. Julian recounts Lincoln using a similar story about a calf in Reminiscences of Abraham Lincoln by distinguished men of his time (1909), p. 241: "There are strong reasons for saying that he doubted his right to emancipate under the war power, and he doubtless meant what he said when he compared an Executive order to that effect to 'the Pope’s Bull against the comet.' In discussing the question, he used to liken the case to that of the boy who, when asked how many legs his calf would have if he called its tail a leg, replied, 'Five,' to which the prompt response was made that calling the tail a leg would not make it a leg."
A very similar riddle about cow legs was also circulated by Edward Josiah Stearns' Notes on Uncle Tom's Cabin (1853), p. 46: '"Father," said one of the rising generation to his paternal progenitor, "if I should call this cow's tail a leg, how many legs would she have?" "Why five, to be sure." "Why, no, father; would calling it a leg make it one?"'
Misattributed

Letter after joining the Army (1939), quoted by Peggy Noonan in "From 'Eternity' to Here" in The Wall Street Journal (25 May 2006) http://www.opinionjournal.com/columnists/pnoonan/?id=110008422

Conversation in Helsinki (2000)

"Lowside of the Road", Mule Variations (1999).

On having his world records beaten
Jesse Owens, Champion Athlete (1990)

Most Famous Motivational Quotes by Dhirubhai Ambani https://www.imagenestur.com/2020/02/dhirubhai-ambani-quotes.html
From interview with Chitralekha

"Forest and Desert," p. 45
The Shape (2000), Sequence: “Bells”