Sahih Bukhari Volume 002, Book 013, Hadith Number 015.
Narated By 'Aisha : AbdurRahman bin Abi Bakr came holding a Siwak with which he was cleaning his teeth. Allah's Apostle looked at him. I requested Abdur-Rahman to give the Siwak to me and after he gave it to me I divided it, chewed it and gave it to Allah's Apostle. Then he cleaned his teeth with it and (at that time) he was resting against my chest.