Musannaf Ibn Abi Shayba 1:1801

Kuthayyir ibn Hisham narrated to us, from Ja'far ibn Burqan, who said: Yazid ibn al-Asamm narrated to us, saying: The siwak (tooth stick) of Maymunah bint al-Harith, the wife of the Prophet, used to be soaked in water. If work or prayer occupied her from it, she would leave it; otherwise, she would take it and use it to clean her teeth.