Musnad Abu Ya'la al-Mawsili 3:335

And through the same chain, from Umm Musa, she said: Abdullah ibn Mas'ud was mentioned in the presence of Ali, and he mentioned his virtues. Then he said: "He once climbed a tree to gather [fruit/wood] for his companions, and his companions laughed at the thinness of his legs. So the Messenger of Allah (peace and blessings of Allah be upon him) said: 'What are you laughing at? By the One in Whose hand is my soul, they are heavier on the scale on the Day of Resurrection than Mount Uhud.'"