I just found this page of Robert Heinlein quotes, many of which I have copied into Evernote. The speaker below is actually a man called Jerry Pournelle, who, in speaking about Heinlein, also quotes him.
When I finally decided to get out of politics, academia, and the aerospace industry and try my hand at writing, Mr. Heinlein was enormously helpful. Years later, when I was an established writer, I asked him how I could pay him back.
"You can’t," he said. "You don’t pay back, you pay forward." I never forgot that, just as I never forgot the wonderful things his ‘juvenile’ stories did for me.
"You don’t pay back, you pay forward." When we went to live in Italy with no money and two children, if natural selection had had full sway, we would have been extinct within weeks so gormless was I. Darwinian forces were held at bay by many people, some of whose acts of kindness were prolonged and profound. And of course, being gormless, broke and ignorant, I could never have paid them back. Not then, not ever.
The kindnesses could not be returned, but they could be passed on. And on and on. The genealogy of kindness is not direct like that of revenge, but it travels much further.