The reason that you are having difficulty locating this quotation in Shakespeare is that it was not actually taken from Shakespeare, although it is occasionally misattributed to him. The quotation is actually from Oliver Wendell Holmes' Autocrat of the Breakfast Table. The work, which was originally published in book form in 1858, is actually a collection of Holmes' essays which were originally published in the magazine The Atlantic Monthly. They are in the format of one-sided conversations occurring over breakfast at a boardinghouse and are notable for their conversational tone.
Oliver Wendell Holmes Sr. (1809 – 1894) was an American doctor and writer from a wealthy New England family who attended Phillips Academy and Harvard College and obtained his medical degree from Harvard Medical School in 1836. He taught at Dartmouth and Harvard medical schools and was distinguished by his insistence that doctors, by poor hygienic practices, could spread puerperal fever from patient to patient (a belief that proved correct). During his long and distinguished medical career he also wrote and published poetry, fiction, and essays.