One such place is Matthew 6:25:
For this reason I say to you, do not be anxious for your life, as to what you shall eat, or what you shall drink; nor for your body, as to what you shall put on. Is not life more than food, and the body than clothing?
'For this reason'? For what reason? For that you need to go back one verse:
No one can serve two masters; for either he will hate the one and love the other; or he will hold to one and despise the other. You cannot serve God and mammon.
Now this is a little scary. If 'You cannot serve God and mammon' logically leads to "don't be anxious about even such mundane, everyday concerns as food, drink, and clothing", it would seem to follow that worrying about such things constitutes serving mammon. And serving mammon will keep you from serving God.
Note that this isn't a question of God arbitrarily deciding that if you serve mammon, He won't let you serve Him. It's not 'You may not serve both God and mammon', as though God was laying down a (hopefully waivable) entrance requirement for the Serving God Club. It's 'You cannot serve both God and mammon'. The thing jest ain't possible. If you're serving mammon, you don't have the ability to serve God, no matter how much you may want to. Which means "Don't be anxious about food, drink, and clothing" isn't some high (and, to many of us, irritating) spiritual ideal attainable only by the most advanced Christian. It's informing us of a basic practical fact: worrying about where your food, drink and clothing will come from will rob you of the ability to serve God. If you want to serve God, you will have to deal with this.