Musnad Abu Ya'la al-Mawsili 152:297

Abu Khaythama narrated to us, Affan narrated to us, Hammad bin Salamah narrated to us, Asim bin Bahdalah narrated to us, from Zirr bin Hubaysh, from Abdullah bin Mas’ud: That he was gathering a *siwak* (tooth-stick) from the *arak* tree for the Messenger of Allah, may Allah bless him and grant him peace. The wind was causing him to sway, and there was thinness in his shins, so the people laughed. The Prophet, may Allah bless him and grant him peace, said: "What makes you laugh?" They said: "The thinness of his shins." He said: "By the One in Whose hand is my soul, they are heavier in the scale than Uhud."