Full Idea
Wimsatt and Beardsley claimed that either the intention succeeded, so one does not need to look outside the work for its meaning, or the intention failed, so external evidence does not help.
Gist of Idea
Intentions either succeed or fail, so external evidence for them is always irrelevant
Source
report of W Wimsatt/W Beardsley (The Intentional Fallacy [1946]) by Stephen Davies - The Philosophy of Art (2nd ed) 5.3
Book Reference
Davies,Stephen: 'The Philosophy of Art (2nd ed)' [Wiley Blackwell 2016], p.114
A Reaction
Actually, the external evidence may tell you much more clearly and accurately what the intention was than the work itself does. The best example may be the title of the work, which is presumably outside the work.