Twelfth Night Allusions

The following lines are thought to be an allusion to...

"Sir Toby: for, as the old hermit of Prague, that never saw pen and ink, very wittily said to a niece of King Gorboduc, "That that is is" ; so I, being master Parson, am master Parson; for,what is "that" but "that"; and "is" but "is"? (IV.ii.15-19)"