From Sahih Bukhari (Hadith 890):
Narrated `Aisha:`Abdur-Rahman 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 Messenger (ﷺ). Then he cleaned his teeth with it and (at that time) he was resting against my chest
Reference: Bukhari 890