In its original form in his Greek work Aphorisms, (sect. I, no. 1) it is “Ὁ βίος βραχύς, ἡ δὲ τέχνη μακρή” rendered in Latin as “Ars longa, vita brevis" and often translated as “Life is short, [the] art long." Or in its full form "Life is short, [the] art long, opportunity fleeting, experience misleading, judgment difficult. The physician must not only be prepared to do what is right himself, but also to make the patient, the attendants, and externals cooperate.” It is also famously quoted, slightly differently, by Seneca in his De brevitate vitæ (On the shortness of life).
Also, famously, the phrase is put to use by Longfellow in his Psalm of Life:
Art is long and time is fleeting, / And our hearts, though stout and brave, / Still like muffled drums are beating / Funeral marches to the grave.