Musannaf Ibn Abi Shayba 27:594

Muhammad ibn Fudayl narrated to us, from Mughirah, from Umm Musa, who said: I heard Ali saying: The Messenger of Allah (peace and blessings of Allah be upon him) commanded Ibn Mas’ud to climb a tree and bring him something from it. His companions looked at the thinness of his shins and laughed at them. The Prophet (peace and blessings of Allah be upon him) said: "What makes you laugh? By the One in Whose hand is my soul, they are heavier in the scale than Uhud."