Musnad Ahmad 5:357

Muhammad ibn Fudayl narrated to us, Mughirah narrated to us, from Umm Musa, who said: I heard Ali saying: The Prophet, may Allah bless him and grant him peace, commanded Ibn Mas’ud, so he climbed a tree. He commanded him to bring him something from it. His companions looked at the leg of Abdullah ibn Mas’ud when he climbed the tree, and they laughed at the thinness of his legs. The Messenger of Allah, may Allah bless him and grant him peace, said: "What are you laughing at? The leg of Abdullah is heavier on the Scale on the Day of Resurrection than Uhud."