Only fools and horses

although this is in the opening song of the BBC comedy - 'Only Fools and Horses', where does the saying 'Why do only fools and horses work', come from. Is it a saying?

Yes, the saying is simply "Only Fools and Horses Work", an old cockney saying implying that if you're smart you'll find a way of making a living without resorting to hard graft. Only simpletons and dumb beasts would actually WORK for a living.