מדרש רבה

מדרש בראשית רבה פרשת וירא פרשה נ"ה סימן ב

English summary:
Basically, the idea is that Hashem only tests Tzadikim / people who are strong enough to withstand the נסיון. The מדרש brings 3 משלים:
1) ר' יונתן: It's like a פשתני (someone who works with פשתן - flax) -- he hits the flax to straighten it out, soften it, and get all the dirt out. But he only hits the good-quality flax, in order to improve it. He doesn't hit the bad-quality flax, because it's not worth it -- it will never be good-quality.
2) ד"א: ר' יונתן: It's like a potter who is checking which כלים came out well and sturdy. He doesn't hit the ones that are weak, because he knows they will just shatter completely after being hit once. But he hits the sturdy ones many times, to make sure they really are strong and can withstand everything.
3) ר' אלעזר: It's like someone who has 2 cows -- one weak and one strong. When he wants to plow his field, of course he's going to put the yoke on the strong one and use the strong cow to plow.
נמשל: So too, Hashem only sends נסיונות to people who are strong enough to handle it...

The מדרש is much much more beautiful in Hebrew, so here: (thanks to HebrewBooks.org for the text ;-))

(For those of you who were in Rav Lubarsky's Seminar class last year - we learned this Midrash.)