Allgemeine Theorie der Beschäftigung, des Zinses und des Geldes
Keynes summarized Say's law as "supply creates its own demand", or the assumption "that the whole of the costs of production must necessarily be spent in the aggregate, directly or indirectly, on purchasing the product" (from chapter 2 of his "General Theory"). See the article on The General Theory of Employment, Interest and Money for a summary of Keynes's view.