Smiles copied his anecdote about Michelangelo from Charles Colton, who wrote:
That writer who aspires to immortality, should imitate the sculptor, if he would make the labours of the pen as durable as those of the chisel. Like the sculptor, he should arrive at ultimate perfection, not by what he adds, but by what he takes away; otherwise all his energy may be hidden in the superabundant mass of his matter, as the finished form of an Apollo, in the unworked solidity of the block. A friend called on Michael Angelo, who was finishing a statue; some time afterwards he called again; the sculptor was still at his work; his friend looking at the figure, exclaimed, you have been idle since I saw you last; by no means, replied the sculptor, I have retouched this part, and polished that; I have softened this feature, and brought out this muscle; I have given more expression to this lip, and more energy to this limb: Well, well, said his friend, but all these are trifles; it may be so, replied Angelo, but recollect that trifles make perfection, and that perfection is no trifle.
Charles Caleb Colton (1822). Lacon: or Many Things in Few Words, p. 94. London: Longman. Spelling modernized.
You’ll see that the text starting “I have retouched this part” is repeated almost word for word in Smiles. Colton’s book of anecdates and aphorisms was immensely popular in the early 19th century, so Smiles may have expected his readers to recognize the origin of the story without needing to be told.
Colton, like Smiles, gives no source for this anecdote, and my opinion is that Colton invented it. I have two reasons for thinking this.
First, the moral of the story (“trifles make perfection, and perfection is no trifle”) is in the form of a chiasmus, a rhetorical device in which two words or phrases are repeated, but in the opposite order. This device was a particular favourite of Colton’s, and other examples from Lacon include:
We should have a glorious conflagration, if all who cannot put fire into their works would only consent to put their works into the fire. [p. viii]
Friendship often ends in love; but love in friendship—never. [p. 58]
When you have nothing to say, say nothing [p. 99]
Liberty will not descend to a people, a people must raise themselves to liberty [p. 123]
they were the very reverse of their own mirrors; for the one reflected, without talking, but the other talked without reflecting. [p. 144]
he that has a sword, will either use it without threatening, or threaten without using it. [p. 151]
Second, nothing like this anecdote appears in either of the biographies of Michelangelo that were published in his lifetime, neither in Lives of the Most Excellent Painters, Sculptors, and Architects (1550) by Giorgio Vasari, nor in The Life of Michelagnolo Buonarroti (1553) by Ascanio Condivi. The closest approach to Colton is this anecdote in Vasari:
It happened at this time that Piero Soderini, having seen it [the statue of David] in place, was well pleased with it, but said to Michelagnolo, at a moment when he was retouching it in certain parts, that it seemed to him that the nose of the figure was too thick. Michelagnolo noticed that the Gonfalonier [i.e. Soderini] was beneath the Giant, and that his point of view prevented him from seeing it properly; but in order to satisfy him he climbed upon the staging, which was against the shoulders, and quickly took up a chisel in his left hand, with a little of the marble-dust that lay upon the planks of the staging, and then, beginning to strike lightly with the chisel, let fall the dust little by little, nor changed the nose a whit from what it was before. Then, looking down at the Gonfalonier, who stood watching him, he said, “Look at it now.” “I like it better,” said the Gonfalonier, “you have given it life.” And so Michelagnolo came down, laughing to himself at having satisfied that lord, for he had compassion on those who, in order to appear full of knowledge, talk about things of which they know nothing.
Giorgio Vasari (1550), ‘Life of Michelagnolo Buonarroti: Painter, Sculptor, and Architect of Florence’ from Lives of the Most Execellent Painters, Sculptors and Architects. Translated by Gaston du C. de Vere (1912).
But Vasari’s story is quite different in tone and moral from Colton’s. Perhaps Colton had read this story and adjusted it to fit his own moral.
(Vasari’s story is also likely to be a rumour or invention. In his account of Michelangelo’s sculpting of David, Vasari gets various details wrong, for example he says that it was Simone da Fiesole who had abandoned the roughed-out block of marble which Michelangelo used, whereas the modern consensus is that was Agostino di Duccio. This suggests that he was not using a reliable source for this part of his biography.)