Verses with the word plumbline in the Old Testament (2 verses):

Amos 7:7
Thus he shewed me: and, behold, the LORD stood upon a wall made by a plumbline, with a plumbline in his hand.

Amos 7:8
And the LORD said unto me, Amos, what seest thou? And I said, A plumbline. Then said the LORD, Behold, I will set a plumbline in the midst of my people Israel: I will not again pass by them any more: