There are 235 verses in Matthew that are paralleled in Luke but not in Mark or John.
This number represents more than a fifth of Matthew and Luke, and so some scholars have proposed that there was a written source—called Q—that both Evangelists drew upon, though it is now lost.
There are, of course, other possibilities. One is that Matthew simply used Luke; another is that Luke used Matthew.
It is possible that they both used a lost written source for this material, but...