Full Idea
'What will be gained by a man who is good?' 'That is easy - he will be happy'.
Clarification
'Happy' is the Greek word 'eudaimon', also sometimes translated as 'flourishing'
Gist of Idea
If a person is good they will automatically become happy
Source
Plato (The Symposium [c.373 BCE], 205a)
Book Reference
Plato: 'The Symposium', ed/tr. Hamilton,W [Penguin 1951], p.84
A Reaction
Suppose you tried to assassinate Hitler in 1944 (a good deed), but failed. Happiness presumably results from success, rather than mere good intentions.