In Psalm 137, on what did the exiles in Babylon hang their harps?

Answer: The willows

They hung their harps on the willows, for their captors demanded, "Sing us a song of Zion."

“There on the willows we hung our harps, for there our captors requested a song; our tormentors demanded songs of joy: “Sing us a song of Zion.””— Psalm 137:2-3 (BSB)