(Numbers 13:16:) "AND MOSES CALLED HOSHEA BEN NUN JOSHUA." And what did Moses see <fit> to add a yod to the name of Joshua? Rather, Caleb took his reward from the land, as it is said (Joshua 14:9): "SURELY THE LAND ON WHICH YOUR FOOT HAS TROD <SHALL BE AN INHERITANCE FOR YOU>," and so on. And Joshua <took> the reward of the ten spies, so that a yod, <which stands for> ten, was added to his name. <Therefore> "AND MOSES CALLED HOSHEA BEN NUN JOSHUA." [Another interpretation:] When Moses saw that they were very wicked, Moses said to him: May Yah save you from this evil generation.
Why Moses Added a Yod to Hoshea to Make Joshua
Midrash Tanchuma Buber, Sh'lach 9:1
ויקרא משה להושע בן נון יהושע (שם). ומה ראה משה להוסיף על שמו של יהושע יו"ד, אלא כלב נטל שכרו מן הארץ, שנאמר אם לא הארץ אשר דרכה רגלך בה וגו' (יהושע יד ט), ויהושע [נטל] שכר עשרת מרגלים שתוסף על שמו יו"ד עשרה, ויקרא משה להושע בן נון יהושע. [ד"א] כיון שראה משה שהם רשעים הרבה, א"ל משה י"ה יושיעך מן הדור הרע הזה.