The desire for safety stands against every great and noble enterprise.
nisi impunitatis cupido retinuisset, magnis semper conatibus adversa.
Original text (Latin)
Notes from Wikiquote
Book XV, 50, in his account of Subrius Flavus’ passing thought of assassinating Nero while the emperor sang on stage.
Variants
"but desire of escape, foe to all great enterprises, held him back."