From Giorgio Vasari, Lives of the Artists (1912 translation), pp. 96-97:

[Leonardo da Vinci] also painted in Milan, for the Friars of S. Dominic, at S. Maria delle Grazie, a Last Supper, a most beautiful and marvellous thing…

It is said that the Prior of that place kept pressing Leonardo, in a most importunate manner, to finish the work; for it seemed strange to him to see Leonardo sometimes stand half a day at a time, lost in contemplation, and he would have liked him to go on like the labourers hoeing in his garden, without ever stopping his brush. And not content with this, he complained of it to the Duke, and that so warmly, that he was constrained to send for Leonardo and delicately urged him to work, contriving nevertheless to show him that he was doing all this because of the importunity of the Prior. Leonardo, knowing that the intellect of that Prince was acute and discerning, was pleased to discourse at large with the Duke on the subject, a thing which he had never done with the Prior: and he reasoned much with him about art, and made him understand that men of lofty genius sometimes accomplish the most when they work the least, seeking out inventions with the mind, and forming those perfect ideas which the hands afterwards express and reproduce from the images already conceived in the brain.