Musnad Ahmad 1159:5

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, he said: Muhammad ibn Talhah ibn Yazid ibn Rukanah narrated to me, from Salim ibn Abdullah ibn Umar, from Abu al-Jarrah, the freed slave of Umm Habibah, from Umm Habibah, that she narrated to him, she said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Were it not that I would burden my nation, I would have commanded them to use the siwak (tooth stick) at every prayer, just as they perform ablution."