Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "I saw myself (in a dream) using the siwak, and then two men came to me, one of them older than the other. I handed the siwak to the younger of the two, and it was said to me: 'Give it to the elder.' So I gave it to the elder."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 171