
“Who kindly sets a wand'rer on his way
Does e'en as if he lit another's lamp by his:
No less shines his, when he his friend's hath lit.”
Homo qui erranti comiter monstrat viam,
Quasi lumen de suo lumine accendat facit;
Nihilo minus ipsi lucet, cum illi accenderit.
As quoted by Cicero in De Officiis, Book I, Chapter XVI - translation by Walter Miller