It should be observed, that one example is taken from royal persons, as David, the other from priestly, as those who profane the sabbath for the service of the Temple, so that much less can the charge concerning the rubbing the ears of corn attach to Him who is indeed King and Priest.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Quaest in Matt., q. 10