Musnad Ahmad 26:441

'Abd al-Samad and Hasan ibn Musa narrated to us, they said: Hammad narrated to us, from 'Asim, from Zirr ibn Hubaysh, from Ibn Mas'ud, that he was gathering *siwak* (tooth-stick) from the *arak* tree, and he had thin legs. The wind began to sway him, so the people laughed at him. The Messenger of Allah (peace and blessings of Allah be upon him) said: "What are you laughing at?" They said: "O Prophet of Allah, at the thinness of his legs." He said: "By the One in Whose hand is my soul, they are heavier on the Scale than Uhud."