Amos 7:7-8

7Thus He showed me [a vision], and behold, the Lord was standing by a vertical wall with a plumb line in His hand [to determine if the wall was straight or if it needed to be destroyed].

8The Lord said to me, “Amos, what do you see?” And I said, “A plumb line.” Then the Lord said, “Behold, I am setting a plumb line [as a standard] Among My people Israel [showing the defectiveness of the nation, requiring judgment]. I shall not spare them any longer. [The door of mercy is shut.]