Musnad Ahmad 27:1766

Ya'mar ibn Bishr narrated to us, Abdullah—meaning Ibn al-Mubarak—narrated to us, he said: Usamah ibn Zayd said: Nafi' narrated to me, that Ibn Umar said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) while he was using the tooth stick (siwak), and he gave it to the eldest of the people and said: "Indeed, Gabriel (peace and blessings of Allah be upon him) commanded me to give to the eldest."