Numbers 13:25
And they returned from searching of the land after forty days.
Cross-reference
Numbers 14:33
Parallel
Numbers 14:33 shows the consequence: the 40 days of spying become 40 years of wandering for the people's faithlessness.
Numbers 14:34
Citation
Numbers 14:34 explicitly links the 40 days of spying to 40 years of punishment, a day for a year.