Sayyiduna Ibn Umar (may Allah be pleased with them both) said: I saw the Messenger of Allah (peace and blessings be upon him) using the siwak; then he (peace and blessings be upon him) gave it to the eldest of the people and said: "Jibril (peace be upon him) ordered me to give the siwak to the elder."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 172