Sahih Bukhari Volume 004, Book 053, Hadith Number 332.

Narated By Ibn Abu Mulaika : 'Aisha said, "The Prophet died in my house on the day of my turn while he was leaning on my chest closer to my neck, and Allah made my saliva mix with his Saliva." 'Aisha added, "'AbdurRahman came with a Siwak and the Prophet was too weak to use it so I took it, chewed it and then (gave it to him and he) cleaned his teeth with it."