Musnad Abu Ya'la al-Mawsili 219:6

Abu Khaythama narrated to us, Ya'qub ibn Ibrahim narrated to us, my father narrated to us, from Ibn Ishaq, who said: Muhammad ibn Talha narrated to me, from Salim ibn 'Abd Allah, from Abu al-Jarrah, the freed slave of Umm Habibah, from Umm Habibah, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Were it not that I would make it difficult for my nation, I would have commanded them to use the siwak (tooth stick) at every prayer, just as they perform ablution."