The story
First attested in William Camden's Remaines of a Greater Worke, Concerning Britaine (1605) as “All the proofe of a pudding, is in the eating.”
Etymology from Wiktionary, CC BY-SA 4.0.
the proof of the pudding is in the eating
Today it means: One can only claim that something is a success after it has been tried out or used. Full definition →