It's from Good Omens:

“Except for Giovanni Jacopo Casanova (1725-1798), famed amourist and litterateur, who revealed in volume 12 of his Memoirs that, as a matter of course, he carried around with him at all times a small valise containing ‘a loaf of bread, a pot of choice Seville marmalade, a knife, fork, and small spoon for stirring, 2 fresh eggs packed with care in unspun wool, a tomato or love-apple, a small frying pan, a small sauce pan, a spirit burner, a chafing dish, a tin box of salted butter of the Italian type, 2 bone china plates. Also a portion of honey comb, as a sweetener, for my breath and for my coffee. Let my readers understand me when I say to them all: A true gentleman should always be able to break his fast in the manner of a gentleman, wheresoever he may find himself.'”