It is narrated from the Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), that when the Prophet (peace and blessings be upon him) used the siwak, he (peace and blessings be upon him) would give me the siwak to wash it. I would first use it myself, then wash it and return it to him (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 169