A group of 12 went into the land to scope things out. 10 of the 12 bring back reports of walled cities and strong men that look like giants so the congregation of Israel decides to appoint new leaders that will take them back to Egypt. This is where the 40 years in the wilderness is mentioned for the first time.

Numbers 14

All the congregation lifted up their voice, and cried; and the people wept that night.

2 And all the children of Israel murmured against Moses and against Aaron: and the whole congregation said to them “Would God that we had died in the land of Egypt! or would God we had died in this wilderness!

3 Why has the LORD brought us to this land, to fall by the sword, that our wives and our children should be prey? Would it not be better for us to go back to Egypt?”

4 And they said one to another “Let us choose a leader and return to Egypt.”

5 Then Moses and Aaron fell on their faces before all the assembly of the congregation of the children of Israel.

6 And Joshua the son of Nun, and Caleb the son of Jephunneh, two of them that explored the land, tore their clothes.

7 They spoke to all the company of the children of Israel, saying “The land, which we passed through to scout it out, is an exceedingly good land.

8 If the LORD delights in us, then he will bring us into this land, and give it to us; a land which flows with milk and honey.

9 Only do not rebel against the LORD, nor fear the people of the land; for they are bread for us: their defence is departed from them, and the LORD is with us. Fear them not.”

10 When all the congregation said to stone them with stones, the glory of the LORD appeared in the tent of meeting before all the children of Israel.

11 And the LORD said to Moses “How long will this people provoke me? How long will it be before they believe me, for all the signs that I have shown them?

12 I will smite them with pestilence and disinherit them, and will make of you a greater nation and mightier than they.”

13 But Moses said to the LORD “Then the Egyptians shall hear of it, (for you brought this people up in your might from among them.)

14 And they will tell the inhabitants of this land: for they have heard that you LORD are among this people, that you LORD are seen face to face, and that your cloud stands over them, and that you go before them, by day in a pillar of a cloud, and in a pillar of fire by night.

15 Now if you shalt kill all this people as one man, then the nations which have heard of your fame will speak, saying:

16 ‘Because the LORD was not able to bring this people into the land which he swore to them, he has slaughtered them in the wilderness.’

17 And now, I beg you, let the power of my LORD be great, according to what you have spoken, saying:

18 ‘The LORD is slow to anger, and full of great mercy, forgiving wrongdoing and rebellion, and by no means clearing the guilty, visiting the sins of the fathers on the children to the third and fourth generation.’

19 Pardon, I beg you, the guilt of this people according to your great mercy, as you have forgiven this people, from Egypt even until now.”

20 And the LORD said “I have pardoned according to your word:

21 But as sure as I live, all the earth shall be filled with the glory of the LORD.

22 Because all those men which have seen my glory, and my miracles, which I did in Egypt and in the wilderness, and have tempted me now these ten times, and have not obeyed my voice;

23 Surely they shall not see the land that I swore to their fathers, neither shall any of them that provoked me see it.

24 But my servant Caleb, because he has another spirit with him, and followed me fully, I will bring him into the land he went into; and his offspring shall possess it.

25 Since the Amalekites and the Canaanites dwelt in the valley, turn back tomorrow and go into the wilderness by the way of the Red sea.”

26 The LORD spoke to Moses and Aaron, saying:

27 “How long shall I put up with this evil congregation, which grumbles against me? I have heard the complaints of the children of Israel, which they grumble against me.

28 Say to them ‘As truly as I live, says the LORD, as you have spoken in my ears, so will I do to you.

29 Your carcases shall fall in this wilderness; and all that were numbered of you, according to your whole number, from twenty years old and upward which have grumbled against me.

30 Without doubt you shall not come into the land which I swore to make you dwell in, except for Caleb the son of Jephunneh, and Joshua the son of Nun.

31 But your little ones, which you said would be a prey, I will bring them iin, and they shall know the land which you have despised.

32 But as for you, your carcasses  shall fall in this wilderness.

33 And your children shall be shepherds in this wilderness for forty years, and suffer for your unfaithfulness, until your carcasses are wasted in the wilderness.

34 After the number of the days in which ye searched the land, 40 days, a year for each day, you shall bear your guilt 40 years, and you shall know my displeasure.

35 I the LORD have said, I will surely do it to all this evil congregation that are gathered together against me. In this wilderness they shall be consumed, and there they shall die.”

36 And the men that Moses sent to search the land, who returned and made all the congregation to grumble against him, by bringing up a slander against the land,

37 Those men that brought up the evil report against the land, died by the plague before the LORD.

38 But Joshua the son of Nun, and Caleb the son of Jephunneh, who were of the men that went to search the land, remained alive. 

39 Moses told these sayings to all the children of Israel: and the people mourned greatly.

40 They rose up early in the morning, and went up to the top of the mountain, saying “We are here now, and will go up to the place which the LORD has promised, for we were wrong.” 

41 And Moses said “Why do you still transgress the commandment of the LORD? It shall not prosper.

42 Do not go, so that you are not struck down before your enemies. For the LORD is not among you.

43 The Amalekites and the Canaanites are in front of you, and you shall fall by the sword, because you turned away from the LORD, therefore the LORD will not be with you.”

44 But they presumed to go up to the hill top: nevertheless the ark of the covenant of the LORD, and Moses, did not depart from the camp.

45 Then the Amalekites came down, and the Canaanites which dwelt in that hill, and struck them, and scattered them, all the way to Hormah.

See also: Numbers 14 in ESV, HCSB, NASB and NIV via Bible Gateway.