Rama had promised Sugriva, the monkey king, that he will defeat Vali for him. But Sugriva doubted whether Sri Ram will be able to defeat the powerful Vali. To give an idea about the strength of Vali, Sugriva narrates him the story of how Vali defeated demon Dhundubhi. Sugriva then gives two tests to Bhagvan.
First test is to lift the skeleton of Dundubhi with foot and fling it to a distance. Rama walked up to the huge skeleton of Dundhubhi and lifted it with his toe and flung to a huge distance.
Next Sugriva asked Him to pierce seven sala trees with seven arrows as this feat was performed by Vali.
Sri Ram took a single arrow. He pulled the single arrow and released it. It pierced all the seven sala trees and returned to the quiver of Rama.
Sugriva was jubilant and his doubts regarding the strength of Rama were cleared.
This story is found in the Kishkinda Kanda of Ramayana.