I heard various rabbonim say that Hashem doesn't give a person test that he can't overcome.
Where is the source for this?
Talmud / avoda zora 3A
midrash / shimois rabba 34
"G-d does not make matters difficult for His creatures; He expects a person to perform according to his capacity. He demonstrated this when He gave us the Torah. Had He come upon Israel with the full might of His strength, we would not have been able to withstand it, as it is written: (Devarim 5:22) 'If we continue to hear [the voice of the L-rd our G-d] any more, then we shall die.' This is the meaning of the verse (Tehillim 29:4) 'The voice of the L-rd resounds with might.' It does not say "with His might," but "with might" according to the might of each individual."
This is also stated in the anonymous medieval work Pischei Shaarei Avodah (ch. 1) attributed by some to Rabbenu Yonah
כי הא-ל ית' אינו מבקש מבני אדם כי אם לפי כחם
"For the blessed God only makes requests from people according to their abilities."
A source is Rabbi Nachman M'Breslov Zatzal לקו"ת מ"ז
זכור תמיד: לעולם לא יושם בפניך מכשול שאין בכוחך להתגבר עליו
Bamidbar Rabbah 21:22:
אֶלָּא כְּשֶׁהוּא נוֹתֵן לָהֶם, נוֹתֵן לָהֶם לְפִי כֹּחוֹ, וּכְשֶׁהוּא מְבַקֵּשׁ אֵינוֹ מְבַקֵּשׁ אֶלָּא לְפִי כֹּחָן
When Hashem gives to His creations, He gives to them according to His means; when Hashem requests from them, He requests according to their means.
Likewise, the Midrash in several places (Bereishis Rabbah 25:3, 40:3, 64:2, Rus Rabbah 1:4) notes that the famine in the days of David HaMelech (Shmuel Beis 21) should have come in the days of Sha'ul, as it was his sins that caused the famine (see further in the Shmuel passage). Nevertheless, it was David's generation that was punished because Sha'ul's generation would't have been able to withstand it, while David's could. The Midrash brings a parable of a peg nailed into the wall meant to hold glassware. To test the strength of the peg, the man hung himself on it first, as he would not be harmed if he fell, before hanging up the delicate glassware.
Chanoch ascribes to Rabbi E. E. Dessler the notion that
everyone always has a spiritual level, and that any seeming challenge not at that level is not considered a challenge for him — it is either something he will certainly do right or something he will certainly do wrong, even if merely out of habit — so he gets no reward or punishment for it.
Logically, then, something one cannot overcome is not a test for him.
I don't know a single mekor but I do know that there are many places throughout the Torah where we see this idea quite clearly. One of my favorite sources is in Parshas Ki Seitzei (21:11) where the Torah permits one to marry an eishes y'fas to'ar. Rashi comments that the reason the Torah permits it is because if it wouldn't, the yetzer harah would drive people to do it in a way that is uncontrollable and therefore cannot be demanded from mankind.
See also: http://thoughtsonparsha.blogspot.com/2014/01/parshasbeshalach-truetests-and.html where the author mentions another "source" - by when the Jews came out of Egypt and Hashem chose not to take them through derech eretz plishtim because - as the Torah itself gives us 'insider information into G-d's thought - He said that taking us that route would result in a challange we as a nation were not ready to face witht he potential to pass. The author states a further application when dealing with expectations of family and friends as could be seen by Yaakov, Yosef, and Moshe.