Musnad Abu Ya'la al-Mawsili 150:400

Abu Ubaydah ibn Fudayl ibn Iyad narrated to us, Malik ibn Sa'id ibn al-Khums narrated to us, al-Sari ibn Isma'il narrated to us, from al-Sha'bi, from Masruq, from Aisha, who said: We used to place the siwak (tooth-stick) of the Messenger of Allah, peace and blessings of Allah be upon him, with his purification water. She said: I said, "O Messenger of Allah, do you not leave the siwak?" He said, "Yes. If I were able to have that from myself at every even-numbered unit of my prayer, I would have done so."