I was not able to find this information anywhere, so am positing the question here. Thank you in advance.
Just checking a non-vowelized page of Chumash like mechon-mamre and doing Ctrl+F, we get Genesis 6:5
וירא ה', כי רבה רעת האדם בארץ, וכל-יצר מחשבת לבו, רק רע כל-היום
God saw that ... all the plans of man's heart were constantly bad.
Of course that's conjugated "libo", "his heart." If you want just "lev", fast-forward to the end of the Flood story, 8:21:
כי יצר לב האדם רע מנעריו
The inclination of man's heart his evil from his youth
Afraid those may not be the uplifting ones you had in mind ... but early Genesis has a lot of human failure ...