I recall learning a midrash (shochar tov?) that the book of Tehillim is out of order and Rebbi Yehoshua ben Levi wanted to put it back in order but was not allowed to do so. Where is this statement found?
Midrash Tehillim on mizmor 3 - see it here
3 lines from the top of the page.
The parshiyos of the Torah were not given in order
R' Yehoshua ben Levy wished to resolve this book (Tehillim).
Referring to Tehillim, continuing on the point above that the Torah was not given in order, the Midrash says that R' Yehoshua wanted to fix it.