In Deuteronomy 1, Moses spoke to a new generation about to enter the Promised Land. He gave them both a history lesson and reminded them of the terms of their covenant with God. He wanted to make sure they didn't repeat the error of their parents, who 38 years before failed to enter the land out of fear of the Canaanites.
That fear was wholly unreasonable in light of all they'd seen over the previous year and a half, proving God's love and power. Moses was dumbfounded that people who'd witnessed so much would balk at a relatively minor challenge of Canaan.
In verse 34, Moses says God heard the people and was angry. We go back to verse 27 to see what God heard and when. There we read, "You complained in you tents, and said, 'Because the Lord hates us, He has brought us out of the land of Egypt to deliver us into the hand of the Amorites, to destroy us.'" I paraphrase: "God wants to destroy us because He hates us." That's more than outrageous; it's blasphemous. It makes God into a devil for it's he who seeks to destroy while God wants us to flourish (John 10:10).
God's anger was due to their thoughts about Him were so utterly undeserved and kept them from enjoying the blessing His love intended.
That tips us off to the origin of this thought. It's ever the accuser (Satan's) strategy to defame God.
We see it back in Eden when he suggested to Eve God was holding out on her, that God did not have hers and Adam's best interest in mind. That lie worked well then, so he's used it ever since.
Take note of where the people were when they said this, in their tents. Their grousing was in private. They complained in their homes. Men to wives, wives to husbands. Mothers to daughter, and brother to brother, they complained about the raw deal God made with them. Though they thought they spoke in secret, God heard them. We ought to pay close attention to our inner monolog, that conversation we all have with ourselves all day and night.
Set a watch on your thoughts.
Some have their origin in the counsels of hell. The Holy Spirit is a marvelous referee in discerning our thought life. It's crucial we pay attention to our inner monolog because it's the seedbed from which our speech comes. And what we say reveals what we believe about God. As Jesus said, it's out of the abundance of the heart, the mouth speaks (Matthew 12:34).
Your mind is the battlefield where the Holy Spirit wars with the world, flesh and devil. The Apostle Paul puts it this way in 2 Corinthians 10:4-5, "The weapons of our warfare are not carnal but mighty in God for pulling down strongholds, casting down arguments and every high thing that exalts itself against the knowledge of God, bringing every thought into captivity to the obedience of Christ."