Verses with the word harden in the New Testament (3 verses):

Hebrews 3:8
Harden not your hearts, as in the provocation, in the day of temptation in the wilderness:

Hebrews 3:15
While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.

Hebrews 4:7
Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts.