It may not be the original, but I feel as if this contributed it to it becoming iconic - the fact that it was in the 1908 Scouting for Boys (the Boy Scouts' manual):

A Good Turn need only be a very small one. It is a Good Turn even if it is only putting a coin into a poor-box, or helping an old woman to cross the street, or making room on a seat for someone, or giving water to a thirsty horse, or removing a bit of banana skin off the pavement. But one must be done every day, and it only counts when you do not accept any reward in return.