Full Idea
Mendelssohn said that what music expresses is not too indefinite to put into words but, on the contrary, it is too definite.
Gist of Idea
Music is too definite to be put into words (not too indefinite!)
Source
Stephen Davies (The Philosophy of Art (2nd ed) [2016], 6.4)
Book Reference
Davies,Stephen: 'The Philosophy of Art (2nd ed)' [Wiley Blackwell 2016], p.141
A Reaction
Not sure whether that is true, but it is a lovely remark. It certainly challenges the naive philosophical view that words are the most precise mode of expression.