Chazal (Sotah 48:) say, "Whoever has bread in his basket and says, 'But what will I eat tomorrow?' he is אמנה מקטני, on a low level of emunah." This is because he wouldn't worry about tomorrow if he had a higher level of emunah. Just as Hashem helped you today, Hashem will surely help tomorrow.
Rebbe Yissachar Dov of Belz zt'l taught another way to read this Gemara. He says that the problem isn't that he is worried about tomorrow. The problem is why he isn't concerned about today! A person who isn't worried about today has low emunah. Although he has bread, that doesn't mean he can eat it. Nothing is guaranteed. One can't do anything without Hashem's aid and should always turn to Hashem to help him.
The Torah writes (25:20), נאכל מה תאמרו וכי השביעית בשנה, "If you will ask what we will eat in the seventh year?" What will we eat if we don't work in the field?
I heard that this question is hinted at in the pasuk (Eichah 3:22), תמנו... לא כי 'ה חסדי, because ו"תמנ is roshei teivos for אכלנ המ אמרת כיו. So, לא ו"תמנ means that we don't ask the question מה נאכל, "What will we eat?" And when we don't ask that question, 'ה חסדי, we will receive Hashem's kindness.
This is because we are better off when we don't ask this question; instead, we trust that somehow, Hashem will support us. The Torah tells us that if people ask, "What will we eat?" השישית בשנה לכם ברכתי את וצויתי, "I will command my blessings on the sixth year," and the crops of the sixth year will last for the sixth, seventh, and even for the eighth year. Rebbe Zusha of Anipoli zt'l (quoted in Noam Elimelech) explains that if we ask the question, Hashem will have to make a new creation, ברכתי את צויתי, a new origin for our parnassah. But if one doesn't ask, the bounty will come down through its original channel and path that Hashem prepared for him.
The Noam Elimelech quotes his brother, Rebbe Zusha: "When Hashem created the world, He created channels to send down the bounty, to fill people's needs. These channels remain open, and the bounty descends. However, when a person doesn't have bitachon due to his worries, the bounty stops, chalilah. Hashem then has to create a new source of bounty.
"This is the meaning of the pasuk, תאמרו וכי; the Torah is teaching that one should have perfect bitachon. He should never ask, מה נאכל, 'What will we eat?' because [due to his question] he is forcing Heaven to toil to make a new command for bounty... Therefore, don't ask questions; trust in Hashem with all your heart, and then the bounty will come to you always, and nothing will be lacking."
