We are obligated to do hishtadlus, and a main source for this is this week's parashah. The Yalkut Shimoni (808) states, "Perhaps Hashem will send you blessings even if you sit idle and do nothing? The pasuk says (2:7), ידיך מעשה בכל ברכך אלקיך 'ה כי, 'Hashem your G-d has blessed you in all that you do.' If a person does, he receives Hashem's blessing. If he doesn’t, he will not receive."
The Mesilas Yesharim (ch.21) explains the purpose of hishtadlus, "Technically, a person could do nothing, and the decree [of how much parnassah he will earn] would take place if it weren't for the קנס, tax, that was placed on all mankind, לחם תאכל אפיך בזעת, 'By the sweat of your brow shall you eat bread.' Due to the decree of the Exalted King, everyone must do hishtadlus for his parnassah. It's a tax that all humanity must pay, and it is one that no one can escape. But it isn't the hishtadlus that helps. It is an obligation. After one did hishtadlus, he fulfilled his obligation, and Heaven's blessings can come. He doesn't need to spend all of his days seeking parnassah."
HaMaspik L'Ovdei Hashem (Rabbeinu Avraham ben HaRambam zt'l, Bitachon 8) writes, "To trust that hishtadlus brings parnassah is כפירה נסתרת (concealed heresy). This is because although he says he believes in Hashem, deep in his heart he thinks parnassah depends on how much hishtadlus he does, and that refuah depends on doctors and medicine. Dovid HaMelech said about such people, שנאתי בטחתי 'ה אל ואני שוא הבלי השומרים, 'I hate those who trust in foolishness. I trust in Hashem.'"
In this week's parashah, Moshe Rabbeinu says (1:12), וריבכם משאכם טרחכם לבדי אשא איכה, "How can I carry all your disputes all by myself?" Rashi says משאכם means they were apikorsim.
In what way were they apikorsim, and why does משאכם mean that they were apikorsim? A translation of משאכם is "your heavy loads." This alludes to those who think they carry the heavy load of life all by themselves. This is apikorsus because they are not alone. Hashem carries the load together with them.
To explain, we share the following popular mashal: A poor man was walking down the road while carrying a heavy package on his shoulders. A king, riding by in his chariot, offered him a ride. The poor man climbed into the wagon but continued carrying the heavy package. The king asked him why he didn’t put his bag down. The poor man replied, "Honorable king. I'm thankful to you for taking me in your wagon. I don't want to trouble you to carry my bag too." The king replied, "I'm carrying your package, regardless, because you and your package are on the wagon. So, you may as well put your bag down." People are similar. Hashem carries them and supplies them with all their needs. If they feel משאכם, that they need to carry their needs on their shoulders, that is apikorsus.
It states in this week's parashah (1:32), ובדבר אלוקיכם 'בה מאמינים אינכם הזה, "In this matter, you do not believe in Hashem, your G-d." The Rebbe of Stretin (Degel Machenah Yehudah) zt'l explains that if a person thinks his parnassah will come הזה בדבר, specifically in this way, and not in any other way, he doesn't believe in Hashem because Hashem can provide parnassah in many ways.
