In Amos's vision, what was the Lord holding as He stood by a wall?

Answer: A plumb line

The Lord stood by a wall with a plumb line, saying He was setting a plumb line among His people Israel and would no longer spare them.

“This is what He showed me: Behold, the Lord was standing by a wall true to plumb, with a plumb line in His hand. “Amos, what do you see?” asked the LORD. “A plumb line,” I replied. “Behold,” said the Lord, “I am setting a plumb line among My people Israel; I will no longer spare them:”— Amos 7:7-8 (BSB)