Duty and pleasure

I found the following written in one of my notebooks. I've no idea whether I wrote it, or if I found it somewhere and copied it out. I could google it, I suppose; but that would be a tiresomely literal minded way of going about things.

Duty has this advantage over pleasure: that whilst doing one's duty is often a pleasure in its own right, that moment when indulging in one's favourite pleasure becomes a duty is the moment the pleasure dies.